The government can't create a public forum and then control what views are expressed in that forum.
Trump could just mute you and no matter what you do he wont be bothered. When he blocks he is taking away your right to critise him on that platform using your own handle.
And if you log back in with the blocked account, and try to tweet @realDonaldTrump, it will prevent you from posting because he blocked you, preventing you from using his Twitter handle in any way.
You also cannot retweet him anymore, which is a common way of publicly condemning the things he says.
In the hands of the President, the block feature is a powerful censorship tool.
When Trump uses his phone to talk to some people but doesn't take my call, is my voice taken away?
Not really. I mean, what did we do before Twitter came along?