Doesn't look like anyone else is tagging them either, so maybe I should move this to reputation instead of scam accusation? My feedback is correct and I don't think countering is really warranted. Opinions?
I wouldn't counter nor would I add a red tag as you mentioned they already fixed the bug. If they left it as it was then I would consider red trust since allowing overmax bets can be "high risk". Your own red trust is probably appropriate because they did take your bet and customer support seems to be confirming that they would have taken the whole amount if you had lost so they should pay the win too.
If you had lost and they took your $300 (or if someone else comes forward with proof of losing over max bet) then I'd say you could add an "implied contract" flag on them.
Overall I'm not happy with their handling of this so I'm adding a neutral to make sure there is a record of some sort even if this particular scenario gets resolved.