When it came to engineering, Scotty was always a straight shooter. He never would have implied that the Excelsior
had transwarp drive only to the degree that his grandmother with wheels would have been a wagon, if it weren't his professional
opinion that the Excelsior
's transwarp drive couldn't work.
Scotty could have been wrong, of course, but it would have been one of the rare occasions. Plus, maybe his contributions, including those he made while captain of Engineering aboard Excelsior,
ultimately ironed things out or led to the solution.
There's just nothing on screen to say one way or the other.