First off, when he says "its a mistake", he means that he doesn't like another country standing up to the US, that is what he meant by those comments. A mistake means "a poor choice or judgement call", so he is correct in his own mind about the definition.
Secondly, Trump is full of hot air here, per usual! This is the same clown that threatened to literally blow up NK 18 months ago if Kim Jung Un didn't agree to stop making nuclear weapons, its par for the course for Trump to behave like this. I find this is how he handles making deals, he starts at the extreme end making the most dire of threats, and then works his way down to the middle ground, I think he feels making these threats will get Iran to agree to drop its nuclear making capabilities.
Thirdly, the US government (and I include past, current and future regimes) totally hypocrites! Its not ok for other countries to shoot down their jets/dromes etc over other countries airspace, yet had Iran flown a drome over the Whitehouse you better believe instantly it would be shot down.