You may be correct, but I'd like to throw out an alternative hypothesis:
With the 12.9 currently MIA at Apple and 3rd party retailers (Best Buy lists the space grey wifi models as "not available"), I think it's possible that we're going to see a minor refresh in the near future. No changes to the design or internal layout, just a swap to the A10X SOC and possibly a true tone display. This means the device can be built without any changes to the production line, just swapping out the logic board and (possibly) display panel. The 12.9 is such a niche product that I doubt it could warrant any other kind of update right now.
The rumored minimal-bezel 10.5 will certainly get an event announcement. I could see both updates being presented at an event, with the minor 12.9 update available immediately and the 10.5 following a month or so after.