(just speculating)
i'm guessing this works because the rimshot is pseudo-squarish in its overall wave form (at least it has a sharp ramp and the volume falls back to 0v or at least below the threshold again before the next hit. quick ADSR), and in this matter maybe acting like a 'clock'? you can do this with a square wave LFO for example, or a 555 timer, or all sorts of things. i mean, you can sync compatible software in the gameboy to external devices through the clock pin on the link port (or perhaps another depending on what the ROM is looking for), its just a matter of figuring out how you need to clock it. depends on what the ROM is looking for, in some respect.
if you're hooking this up to a modular, or anything really, make sure you don't feed it more than 5V (the GB clock pin). you could add a potentiometer and turn it up just enough to get it to sync on your signal, it doesn't have to go up the full 5V just above a trigger threshold and the pot would help out just in case the signal is over 5V
hope that makes some sense
tbh, i'm not sure the frequency range of a 555, perhaps its not appropriate. a LFO with a square wave that has a decent frequency range should get you some results.
but, with a drum machine or something alone those lines, this could be a quick way to add sync without MIDI or anything like this. you are more-or-less replicating the MIDI/sync 24 via audio pulses. audio in this manner is just electricity too, so mimicking the standard syncing technique in a music making device should work fine. you just play it to the link port, instead of through a speaker. devices with individual outs are prime for this sort of thing.
(this is also a poor mans way to sync a variety of things)