"Big tech san francisco" has no obligation to platform anything. They can choose which posts they want to leave up based on their own whims. Now, if the government were to crack down on lab leak speculation, that would be a legal free speech issue.

Slander, meanwhile, is by definition false (as determined by a court). If the court finds that it isn't false, then it wasn't slander. Thus it isn't protected by law, and the state can prosecute you over it.