No, I didn't say that IRS is investigating. I have no idea if they have or are now investigating the hotel or looked at this transaction. Weird you'd make that up.
You said, "I don't know why the IRS would have been involved; is it alleged Donald Trump cheated on his income taxes?" Yes, that's the allegation, and the transaction is income taxable under both federal and state laws.
From what we've read in several cases, he is in fact smart enough to commit tax fraud and not get caught. This transaction is a good example. It's been caught, but something like 7 years after the fact. Why do you think this is surprising?