The law at issue still allows forums with 49,999,999 or fewer users to discriminate based on political viewpoint. It only says sites larger than that need to allow all political viewpoints (but only if they allow any political viewpoints, and only if they have users in Texas).
Legal: Message board with 49 million users that says "no Republicans allowed."
Legal: Message board with 51 million users that says "no politics allowed."
Legal: Message board with 51 million users (none in Texas) that says "no Libertarians allowed."
Not legal: Message board with 51 million users (some in Texas) that says "no Democrats allowed."