Serious, its no joke. He is cut off for life. For Trump this is on par with being forced into bankruptcy or having a bank foreclose on his company.
A large part of Twitter's bottomline is that they don't want Trump to tweet something that leads to something far far worse in the future. Imagine that Trump tweets "Timothy McVeigh was a true American hero. America need more heroes like him that take matters into their own hands"
Sure, Twitter can take it down within minutes, but that is after 70 million people have received the tweet and more than a handful decide the time has come for them to become the next Timothy McVeigh. Maybe next time one of those DC rioters has backpack full explosives.