But it would have been easier if WotC more openly announced the use of new guidlines based on essentials. So they wouldn't look at each product in hope not to see material based on them.
Ever since they started to have races with two options for one of their stats, it became the new norm. I don't recall an announcement about that new guideline (and it was introduced only halfway at first, with Changelings and technically Shifters in the same book as the warforged and kalashtar, having a 50/50 mix of old and new race design).
They introduced racial paragon paths in PHB2, and it became the new standard.
After PHB1, they phased out the "choice of two attack stats" for classes. That is the new standard for all future classes.
In general, every new book has introduced new design concepts, and outside of some [i.e. psionic design is limited to psionics, certain classes have their own special things that other classes don't, like monks with their full disciplines, or the runepriests modal powers, etc]. Essentials is following the same kind of concept. The
players have treated it as a bigger shift, but from the designers perspective, I don't see how they would see a need to annouce a shift in design philosophy. No one expected PHBIII to ignore most of what was "learned" in PHBII, and be more like PHBI. Every release has built off what had come before ... if the books ignored the Essential stuff, THAT would be unexpected, and would probably warrant some kind of mention.