Yeah, I mean they did say that they don't know anything about music and certainly never claimed that this is advanced or anything. But even if it's basic, I mean it's not like this happens in every song. Most music just has a lead melody and everything else playing backup, not several melodies.
Most music actually has several melodies. If you have chords, you automatically have a melody that emerges from the highest note of each chord, and it's common for chord progressions to have melodic elements in the lower notes as well (e.g. a sus4 -> major triad resolution). Even if you don't think that doesn't count because these won't be as prominent as the main e.g. lead vocal melody, it's not at all uncommon for there to be several melodies that are not tied to any instrument playing chords.
Consider the following song, which is a very representative hit pop song of its era, not really known for being particularly complex or advanced, and I picked it because it was the first generic pop song that popped to my mind:
The verses are intentionally minimalistic because that was a part of the aesthetic of late 00s pop music, but the chorus has:
1. the main vocal melody
2. the prominent higher vocal harmony, which, although it mimics the main melody, has very different harmonic tension so it should count as a separate melody
3. the "wooooo~" vocal melody
4. the supersaw synth melody
5. the more percussive synth playing chords and therefore adding the melody of its highest notes, which is different from any of the other melodies
All playing at the same time whenever the wooo~ briefly overlaps with the lyrics. This is a lot more than just one melody, but it's about what you would expect from generic pop music.