Hmmm,... doesn't sound very convincing.
I suspect the reason for this change back to the older version is economic, rather than a supply issue. The V2 uses very similar parts to V1, and so if there was a part sourcing issue, it would affect the whole Triaxis, not just the 'Recto' part,... and probably quite a few of Mesas other products,...
Probably, and this is just a theory, the Recto mode was a pain in the butt (since it involved quite a bit of hotrodding to the original Triaxis), and the addition of a whole new board, complete with it's own EQ. That's a lot of trouble to go to just for one mode. Those who've seen inside a V2 will notice just how large that 'piggyback' board is.
As for all the modes being Mark IV, you can see from the original Triaxis that the whole point of thing was to capture the whole history of the Mark Series in one unit. The Mark series is really the history of Mesa pre-Recto (since all models up until that time were derived from/variation of the Mark theme,... eg. the Quad, the Caliber series etc.). So having everything based on the Mark series kinda makes sense.
Also, in a sense, the Mark IV kinda does that already (captures all previous Mark sounds, except for the Mark I lead stuff), so it's not surprising that a lot of the modes in the Triaxis are related to Mark IV circuits.