Playing samples in patterns and computing coded sound are completely different efforts.

But the site already has tons of tracker music, as well as other not-strictly-chiptunes like an incredible number of .mid versions of classical music and contemporary pop songs of all things.

Tracker files: audio samples as instruments played by a table of notes and variations.

MIDI files: a table of notes and variations for playing default samples for instruments.

Chip synthesis programs: code that generates sequences of sounds, requiring some specific synth engine.

The former two have a radically different (lack of) complexity compared to the latter...

I know, but I don't understand your point. The site is full of .mids and mods, as well as other mod-like formats ripped from various devices. I'm not even sure if it does have any actual chiptunes, as in synthetized in realtime!

You wrote that you find it a «strange omission» not to have included SID tunes in a collection of «mids and mods». Well, simply, the coder must have had an engine capable of playing the latter, which is simple - and not an engine capable of playing the former, which is complex.