The only way the applicable law could be constitutional would be for the law to mandate that the venue not filter ANY content regarding the filtered subject. In other words, Google would have to delete ALL terrorism related posts in order to delete some terrorism related posts on one side or the other. Otherwise Google would be discriminating against chosen speech on behalf of the overall community. Google would be acting as a government, and governmental censorship is unconstitutional.
Seems to me all social media platforms should report and delete ALL terrorism posts they can find. But for posts that are not illicit a non-defacto public square should be able to do whatever they want. If you don't want some annoying guy on your Minecraft server you should be able to kick him off. However if your Minecraft server somehow grows to be a major avenue of political debate than you should lose that right because it would interfere with freedom of speech. If the annoying guy calls for terrorism you should be required to make some reasonable effort to kick him off and report him even a non-annoying guy or anyone. Certainly the effort should be even handed but be aimed at all illicit use, not just some.