I hear ya, Malloy. That's what I thought too. But as I watched the match, Howard Cosell ( yes, he was commentating and he was orgasming over Nastase ) and John Newcombe mentioned this was his first match as a professional. It even said it on screen.
I think McEnroe had been in other tournaments before, most likely as an amateur. They mentioned his year at Stanford and that he was only 18. I'm just quoting what they said.
Correct, it was his first match as a professional, but not his first professional event(which you said in the first post)
McEnroe had played many pro events in the past year(reached the semis at '77 Wimbledon) Enough events to already be ranked in the top 20 at the time of this match. He was seeded 15 at the USO less than 2 months after this match.
This was still early in the open era, amateurs doing well on tour was not that unusual(Pam Shriver would make the final of the USO that year as an amateur)
Also being a great player at 18/19 wasn't that unusual either circa 1978.