I mean, not just impeachable. Criminal, too.
It is, to be sure, but impeachability's the more immediately relevant standard given the impossibility of indicting a sitting president.
The untested and pretty certainly untrue "impossibility" that was pulled out of the ass of Nixon, who subsequently didn't have the balls (or had enough brains to not try) to go through with it. That was kinda' pissed on by the republican treatment of clinton, t'boot.
You're one letter off, I'm afraid; Robert G. Dixon wrote the Sept. 24, 1973 OLC memo
Re: Amenability of the President, Vice President and other Civil Officers to Federal Criminal Prosecution while in Office, in which the President's unique immunity to criminal prosecution was established as OLC opinion. (While they specifically discussed federal prosecution, their arguments remain persuasive regarding prosecution by the states.)
The 2000 version of the memo summarizes it, as well as then-Solicitor General Bork's arguably more structurally compelling agreement on the same grounds regarding Agnew that, to horribly TL;DR it, allowing the judicial process to incapacitate the executive branch violates the separation of powers (and any extension of the executive's power over prosecutions to a case in which the chief executive is the defendant an inherent absurdity) while also running contrary to the national interest, although the President is unique in having enough power personally that his indictment would constitute such an incapacitation.
Far from being "pretty certainly untrue", it's
technically an open question (albeit one with persuasive structural arguments that OLC's opinion summarizes), but as a practical matter you'd be hard-pressed to try, no pun intended, to settle it before the President's term was up.