What stood out to me in the tweet in question, is that he didn't capitalize Trump's name. Usually, professional 'journalists' know how to use proper capitalization.

Can anyone with Twitter look back through his prior tweets and see if he has a habit of not capitalising 'Trump'?

If he never did it before, it doesn't prove he was hacked, but does make it slightly more believable. But if he does have a habit of not capitalising, that's probably proof he wasn't hacked.