It would only be a problem for the account that the purchases were made for. I don't think anyone who stole a credit card number would use those funds on an account they were not using. Her account will be safe.
Considering that accounts are likely to be killed if the owner is found to be using a stolen credit card, the account owner may not get very much out of this. Also there's no re-sale value for the stolen goods. I don't know how Valve works for people selling stuff, but here's a scenario:
1) Create an account to sell stuff via Steam - acting as a games publisher or using a consumer mechanism that allows things to be sold (used games, mods, etc)
2) Compromise an account with a credit card attached, and buy stuff from that account made in step 1
The effectiveness of this plan depends on the fraudster being able to sell things and being able to get the money out of there before Valve can claw it back. Even if it's possible, the odds of someone actually doing this to Wu seem fairly low. Anyone interested in making money is likely to avoid high profile targets. If Wu's account/card were compromised, then I imagine this is someone doing it to piss her off. I think it's more likely, given Wu's tenuous relationship with honesty, that her account is not entirely accurate. For one, how is that every negative thing that every happens in her life is down to this Gamergate person?
Here Wu is claiming to be receiving death threats, so she reports this to the FBI by tweeting it? No, there are oppression points to be earned on Twitter, and death threats are valuable currency.
That tweet mentions having their phone numbers. Should she pass these to the FBI by actually filing a formal complaint? Nah, that'd be awkward, as making false or overstated claims to law enforcement can end badly.
Wu is all about mob outrage. Commanding an army of angry drones is bread and butter for SJW royalty. The only way she could possibly take the higher ground would be in the company of Nazi paedophiles arguing over who gets first dibs on the kid.