In 2017 a girl texted her mother during a home invasion. ABCNews ran a piece about it. The messages included an auto corrected one that read "I'm baby".
A screenshot of that got used for an advert on tumblr, and then tumblr users decided to joke about what it meant because it makes no sense. Eventually it coalesced into a meme that sort of had it as a phrase like I'm an uwu smol bean that can't adult (usually ironic)
At the same time there was another meme, "I'm just a hole, sir", the idea being that you're so horny and so attracted to someone that you'd let yourself be used as a hole for fucking.
Kevin also has a predilection for autistic furry babytalk, which often features traits like dropping pronouns ("give nuzzles, am sad").
So what Kevin did was take those memes and attempt to communicate he's a horny smol bean, thus
However if you don't have Tumblr/Twitter brain rot then the phrase "am hole" is entirely nonsensical and does just read like a single word. Thus the amhole was born, and in relatively early days for the thread. It's the tweet he's arguably most infamous for, as the tweet he's most famous for is that stupid egg cracking one.