As fas as I know, midi is required to have seperate tracks for each instrument.
Wrong. There is absolutely no limitation concerning the usage of
GM tracks in a MIDI file, you can put as many or as few instruments in one track as you want. What you probably are referring to are MIDI
channels, which can indeed only play one instrument at once. But how instruments are assigned to MIDI channels is completely up to the user.
OpenMPT creates a GM track for each mod channel, and that is probably not going to be changed until a full re-write of the MIDI export is going to happen, because I really don't want to dig into that code.