Do people usually sing diatonically?
That notion is so broad that the simple answer is: no.
But, a lot of pop music uses the pentatonic scale or small segements of it like DO-RE-MI or SOL-FA-DO etc. You can think of that as diatonic with the ^4 and ^7 scales degrees omitted. But, this applies to so much music that it is almost a meaningless observation.
Also, it's very common to have blues notes in the melodic mix like flat ^3 or flat ^7 and that of course would not be diatonic.
Pop music can be very diverse from a tonal/harmonic perspective. Personally, I think pop music (which many people may deem simple) requires fairly sophisticated analysis compared to something like a Mozart minuet (which some may consider more sophisticated that pop music.)
I didn't listen to the video, but I think there is a lot of click-bait out there claiming to reveal the "secrets" or "tricks" of music. But often they only present straight forward music theory or they over simplify.
So, Post Malone is not catchy because he sings diatonically. You can prove this to yourself easily... sing something to yourself diatonically... was it an catchy hit? Probably not! Many other musical factors come into play to make something catchy.