I think I was the one who said "not seamlessly" so let me answer.
Well, to me 100% compatible means that essentials and PHBx classes can fully work with each other, but it is not quite the case.
Say for example I make PHBI fighter, and I want one of the knight or slayer 1st level stances.
I cannot get them. At the moment there is no way to get that. In order to multiclass or take a power from another class, the level and type of power must match, but with essentials new format, this is not always the case. Many essentials power lack that power level. On purpose, or not, I am not sure, but I would bet in purpose.
Apparently in an upcoming book there will be rules for more seamless integration of essentials4E and PHB4E.
But do not mind me, really. They will work together in the same party in the same game with only multiclassing or selecting powers from one book to another causing a few problems.