The judgement governs the President's use of Twitter, not the interaction between Twitter and other people. Twitter does not magically become a Public Forum, just because the President uses it.
Now, I'm not sure that is Right and Just, but I'm also not going to worry about it too much. I don't use it enough to get completely wound up about it.
If twitter isnt a public forum, then trump should be able to block whoever he wants.
Yes, including the ability to deny entry to anyone that Trump doesn't want there. But now a judge has ruled they have a right to be there if Trump was the one asking to ban them, but in doing so has created a loophole where as long as I want to deny them entry because Trump doesn't want them to be there (note that this isn't done on behalf of Trump, but because it is my own desire) they can be banned. It makes the judge's ruling extremely easy to work around, which indicates some fundamental problem with the ruling.
Now, twitter banning someone from all other parts of its service wouldn't be impacted. But banning someone from participating in the public forum should no longer be allowed unless done for a reason that the government would be allowed to use as well.