He admitted it himself, he wildly exaggerated to become known as a miracle worker, no exaggeration, no miracle worker, no best engineer.
As already pointed out, and correctly, by Maurice
upthread, that was a joke. Playful banter between two old friends, Kirk and Scotty.
As for, "when was he ever a shown to be a good engineer?" For a question that requires only a single example to answer, how about The Doomsday Machine,
for just one example among scores.