^^^ I've seen plenty of peeling/flaking OEM engine paint through the years, especially after the engines were a few years old.
Especially on the tin.
But not all of them, to be fair.
Whether it was due to chemical strength or cleanliness (which could be doubtful, don't know, wasn't there), I don't know.
I do know that I don't want to paint an engine (or anything else) twice, so I use a primer.
As a bonus, primers generally require less paint application to get the right final color.
Strictly correct? Nope. Looks great for decades? Yup.