Didn't Honda, Chrysler and Ford all nix Dexcool several years ago due to
increased incidence of gasket leaks when testing the stuff? I thought I
read that somewhere 4-5 (or more) years ago.

GM has a bad habit of allowing their corporate ego about a decision get
between them and what the ultimate realities turn out to be (just
personal experience). That could be the case here,...Dexcool be damned!