They made claims and threatened legal action; certainly the public doesn’t need to legally know until a lawsuit is actually brought, but they would be required to present such evidence to the public in court.

Do they have to currently? No, of course not. But this isn’t someone claiming that another did something behind closed doors to bait public disclosure. They opened this door by making the claims, from a standpoint of whether one might want to do business with replit in the future it’s entirely reasonable to ask for proof of what otherwise are slanderous claims.