The first amendment also protects freedom of association. You don't have a right for people to hear you, but the government doesn't have an unlimited power to restrict your ability to actually reach people.
If government prevents people hearing you speak, that's a pretty serious curb on freedom of association.
You have the right to be heard by people that want to hear you.
Nothing says you have to be broadcast or promoted and nothing says you can impose on people who don't want to hear you, but the government telling people they can't talk to certain other people is not the same thing.
Also, in another comment you said "This is also great for hiding and suppressing organization of social uprising. This would have totally suppressed all the BLM content this summer."
Is this sarcasm and you are contradicting yourself or do you actually think it would have been great to suppress BLM organization and protests?
Thats basically the same thing. For example, if the government made newspapers illegal, that would technically be "limiting the audience", but it would still be unconstitutional.
> I think the flaw is to compare old medium to today's technology.
Courts apply laws to new technology all the time. And in this situation, it is very obvious that the courts would rule government restrictions on social media, that make it illegal for people to broadcast on social media, would be unconstitutional.
> What can we try then
Well we can't ban people from using social media, or for broadcasting to many people. That would very obviously be unconstitutional.
You have the right to speak, not the right to be heard.