I think that counts as a secondary/#2 source?

You're not OP

It really depends on how many leaves we're talking; a thin, evenly distributed layer? Yeah that's just mulch and is great. A thicker layer that turns slimy and dense? That grass is a goner. Area and species of leaves probably pays a big part I imagine. I have an area near a fence where the leaves piled up and were left a year and now there's no grass there, even a couple years later (there's a super embedded layer of decomposing leaves that's blocking everything else out even after removing the bulk of the leaves)

Of course, there's never room for nuance in these conversations.

I've got two big sycamores in my front yard, and they both are currently dropping leaves the size of dinner plates in enough quantity to completely cover large portions of the yard. If I don't rake or mulch them, they will smother whatever ground cover that's underneath them. I know this because I tried leaving them one year and it took the next three years to get all the mud pits left behind in the spring to fill back in.