I doubt that was the reason. They announced the DLC a few months after the games came out and considering they showed off, gosh, I think just about every single design except for Zarude (for obvious reasons) & the Horses (& Rider calyrex)? I think it's far more likely the plan was for the Tundra to just not have more g-maxes, instead wanting IoA to be the primary focus on that type of thing (alongside other "battle ready" stuff) while Crown Tundra focused more heavily on legendary pokemon. Calyrex probably could have gotten one, certainly be an interesting theming since it seems to be able to dynamax under its own power, but I think they wanted "just" the Rider to be the gimmick while Urshifu had the G-Max gimmick instead, instead of both.
Also, at risk of being too "humans love making patterns where there are none", there were a roughly equivalent amount of G-Maxes, Galarian forms (and almost the same new Legendary Pokemon but that requires you to not count the birds so *making the iffy hand motion*). I really think they just went "let's get the starters and Urshifu and wrap it up. we're fine with this number of g-maxes"