I get the general idea of doing that, and it makes sense, but it doesn't seem to necessarily match up with what's in ImageMagick's commit history or in their forum post... but would make sense with using the "weird" formats as the initial payload, I suppose.
In particular, ImageMagick accepting MSL directly into convert seems like an extremely straightforward exploit path, so much so that it actually seems unlikely. Their documentation makes it seem like it's designed to use a separate command "conjure," but... some combination of factors is at play here, anyway.
So, given the details now, it seems that the reason the IM commits don't seem to match up is that they didn't really squarely address the problem (in my understanding).
In particular, ImageMagick accepting MSL directly into convert seems like an extremely straightforward exploit path, so much so that it actually seems unlikely. Their documentation makes it seem like it's designed to use a separate command "conjure," but... some combination of factors is at play here, anyway.