The article does not say “when”.
I'm guessing the event will be more successful for President Trump than it was for Governor DeSantis. For one thing almost all rank and file DeSantis voters can be expected to enthusiastically support President Trump but the reverse did not happen earlier.
By your logic, the smaller DeSantis crowd crashed X so why would Trump risk his bigger crowd on X when even his own Truth Social couldn’t successfully host a live event? Yes, it is a risk either way, hopefully Musk has learned and X is up for the challenge.
Still, I think the traffic for a Trump event will be on a whole higher level. They should be prepared for that otherwise the site will crash.