I don't think it's as a big a deal as it is being made out to be. There is always going to be a boom in camera sales following new releases. Sony released 2 full frame cameras since November 2017, while Canon and Nikon released none. Push back further to May, and Sony has released 3 full frame cameras while Canon and Nikon released 1 each. Of course Sony will sell a lot of full frame cameras right after releases when the competing brands have not released anything in the same timeframe.
Looking at Nikon specifically, they announced being number 1 in full frame camera sales following release of the D850 last December and they even based that statement on the same data sources referenced by Sony now. Nikon's lead obviously didn't last long, because the growth in sales was directly linked to a boom in sales for a new model.
Claiming to be first for a short snapshot in time really only tells part of the story. Now, if Sony remained first in market share for full frame cameras over a full release cycle of Canon and Nikon bodies (as in each company gets to re-release their full series of full frame cameras), then that would be some serious bragging rights. Until then though, I would expect each company to cherry pick data that aims to catch headlines.