Firstly, what you've attached here is correct. To buy that much ROBUX straight from ROBLOX themselves would cost around a grand, which is absurd. However, that's not the only way to get ROBUX through the platform, there are actually multiple methods and in a way I respect how it can teach working for a reward to the kids playing the game. First of course, is owning a game. There is a subscription on ROBLOX called Premium, it's the same three price points as above, just per month. The only difference being the amount of ROBUX given to you at the beginning of a month. If I buy the 20 dollar Premium tier, I get 20 dollars of ROBUX per month as my subscription renews, and et cetera. This also means you can make a one time purchase, than cancel the recurring payments to get a little bonus. Why buy 1700 ROBUX for 20 bucks when you can get 2200 for 20 by just cancelling the subscription's automatic renewal immediately after?
Anyways, back on topic. If someone with a Premium ROBLOX account plays your game, you earn a small sum of ROBUX off of that. It increases depending on several factors, such as time spent by the Premium user and the amount of them in the game at once, but generally I've found (although it seems to heavily vary between games and users) that you can earn around a dollar worth of ROBUX from 10 minutes of 1 Premium user playing your game. Sounds like nothing, but when your game is popular and you have multiples of them all spending hours at a time on the game, this can quickly add up. That's also on top of Game Passes and Items, essentially just game-specific (not site-wide of course as ROBLOX is a platform and not a singular game) microtransactions, and to be fair THIS is where the predatory practices come into play. Outright lootboxes/gambling and stamina systems seem to be totally banned as not one of the top-earning/most popular games on ROBLOX have any semblance of those systems, but regardless they can still get pretty overpriced and worthless for the most part. Usually, developer payouts and Game Passes are how users end up in the millions and billions of ROBUX range on the site and end up being able to afford all the rarest and most expensive items.
Then there are commissions. Obviously this works in many different forms but basically, imagine a ROBLOX game developer who doesn't want to model their own lobby, or gun assets or what have you. They can commission an independent builder/scripter/whatever to make it for them, and pay via ROBUX (Usually 1000/2000 or 10 dollars/20 dollars at the cheapest) by purchasing an appropriately priced t-shirt from that user. If you're good enough with LUA (ROBLOX's scripting engine) or Roblox Studio (its proprietary game/model development software) than you can make some serious ROBUX off of just that.
There are also clothing sales. Any user with a Premium subscription can design and sell custom T-Shirts, Shirts, and Pants for other users to buy for ROBUX directly (ROBLOX takes a cut per sale though) and wear on their avatar. This is probably the hardest because clothes are cheap and can take a lot of effort, only for your design to get stolen and sold for cheaper by one of the thousands of clothing resell bot groups. Certain users, such as popular ROBLOX YouTubers and game developers, for instance, can also apply for ROBLOX's UGC (User Generated Content) program to sell their own modelled and textured hats and other accessories for a higher price, however. (Usually only the official ROBLOX staff can create and sell these types of items.)
Last but not least, you'll notice the item in the screenshots is labelled as a Limited item. This ties into ROBLOX's trading community. Any user with an active Premium subscription can trade Limited items (items sold for a limited duration of time) or Limited Unit items (items only sold in a certain amount) with other users who have them. You can also resell these items directly for your own cost (though ROBLOX takes a cut here too). If you want a comparison, I'd say it's closest to TF2's trading system. It's so old at this point that numerous offshoot communities have sprung up around trading Limited items, and really anyone can, with a bit of practice and knowledge, pick up a cheaper common Limited for around 5-10 dollars worth of ROBUX and trade up into having hundreds of thousands of ROBUX worth of items.
Of course, the main thing you may be wondering is "what retard does all of this just for an ingame currency" and that's a valid question. However, it all clicks into place when you consider ROBLOX's Developer Exchange program. You can trade ROBUX you earn from any source directly to ROBLOX for a paycheck proportional to how much ROBUX you trade. Rates fluctuate, though with the current rates the teenage developer of "Jailbreak", one of ROBLOX's most popular games at the time, was able to earn over $3,000,000 USD off of the revenue from the game alone. I also know there are very dedicated ROBLOX traders who will start with 20 bucks at most, trade up until they have hundreds of thousands of ROBUX in rare items, then sell them all off directly on the Avatar Shop and cash out for a couple hundred to, in some cases, even a couple thousand dollars in real USD, than use a bit of that to start the process all over again. (Also no, DevEx rates do not allow you to buy ROBUX and then immediately trade it in for a profit. The rates don't allow for that to be a viable process, you could do it theoretically but you'd lose a fuckton of money.)