You technically cannot save samples > 64k in MOD files
That's not true. The sample length field is 16 bits wide, but the lowest bit of the sample length is not stored. Effectively, that means that you can only save samples whose length is a multiple of 2, and furthermore, it means that you can save samples that are up to 1FFFF bytes (or 128K) instead of FFFF bytes (or 64K) long. Most Amiga trackers don't support sampels that are longer than 64K bytes, but there is nothing wrong with storing them in the file. It is technically perfectly possible, it's just that Amiga trackers can't handle it.