I don't think it's fair to proclaim that it's canon because of a detail in a spin-off manga. It's a theory, yes, but confirmed canon? No.
First off: the spin off in question is canon, second, hypothesis not theory if it wasn't confirmed, third it's not the only proof, only the most blatant and easy to show, and the one I know the most and can refer to the fastest, all the rest has been told by PMMM experts on PMAS.
The design has not been chosen randomly, it is shown quite clearly in several panels to make quite sure you see it, it is very much designed to make you think of Walpurgis, and the design of Walpurgis is very much in line with the time period and region where Isabeau comes from.
Walpurgis is consistently depicted as beingg able to make herself invincible in the games, which is in line with Isabeau's invincibility to magical girls thanks to a wish.
There is exactly one known example of witch conglomerate in all of pmmm, walpurgis, and Isabeau is the only example we have of a witch finding herself in a situation that could lead to one such conglomerate, trapped in a different dimension with quite a lot of other witches.
This also explains why attacking Walpurgis before she appears, during her travel is not possible: she's still in the separate dimension she was trapped in during that time, she just obtained the ability to get out.