It might be an optimum design for a glider, e.g., here is a design for India's Space Shuttle https://dynaimage.cdn.cnn.com/cnn/c_fill,g_auto,w_1200,h_675...

I actually just saw a documentary about the Mig 29, seems like it was a pretty revolutionary design https://youtu.be/zCnpuoD1PLo

Both the Mig-29 and Su-27 are based on the same aerodynamics research. That they look similar to the F-15 is probably more due to the fact that they were all designed to be air superiority fighters than any espionage.

Something that caught my eye was not just the overall layout but the intakes' shape.

When I say it looks like the F-15, I'm saying, as opposed to, say, the F-16 or F-18 or F-14.

And it's just a comment on the superficial appearance, not based on aerospace knowledge or about espionage.

This thread isn't even about espionage, is it?

Maybe you misread something? If I think something obviously looks like something else, that's not something you can disprove with a citation. There's no assumption involved.