I know of an (ex)-CGC sig-witness who scammed people out of their HG books that he would swap out for his own personal lower grade books for sig-series.
He was eventually caught, but no telling how many hundreds of books he did this with before getting caught, and I'm sure he wasn't/isn't the only one.
But in the case of this seller who cracks, swaps, then resubmits for reholder or ME without CGC checking, that's the CGC's sloppiness being exposed and exploited.
And as you said, it's pretty certain this seller isn't the only one to have figured out that CGC's sloppiness is ripe for being exploited.