I'm not a dev, but if this is written from scratch, I suspect support for these old formats would require an entire re-write of the codebase. OpenMPT also already supports all of those formats (to my knowledge), so the functionality would be a bit redundant, within this already niche scene.
Just because the interface looks similar to other trackers, does not mean the software is handling all of the necessary legacy effect commands, or the audio itself, the same way.