No, they could release products that don't have clearly mechanically superior options. Just because you're putting out new books, does not mean that those books have to be full of crap that is just plain better than what came before.
That guards you against only one kind of "Power-creep".
For example, let's pretend there was a feat that gave you +2 damage to attack against prone targets. Not too powerful, I think.
In 4E core rules, a Fighter might take this power. How many powers does he have to knock someone prone? Maybe one or two per tier?
Now fast-forward one "Martial Power" and one PHB II. How many powers that knock targets prone does he have now? So, he can't get a benefit from this +2 damage feat most of the time. But add a Rogue with that feat, and he suddenly deals 2 points extra damage every round!
An even better example might be a Wizard with those cold-enhancing feats. How many cold based powers does a Wizard have access too now, how many powers will he have after "Arcane Power" and PHB II?
Guarding yourself against "synergistic" power-creep is nearly impossible. Maybe it would have worked if there were general design rules that say "only two knock-down powers per tier, one level 1, one level 5". Maybe the 4E designers even have such rules? Which would make the 3PP support and homebrewers guilty of power-creep.
My best guess is that the 4E designers have built the system to already account for most synergies as I described them. If they built a Wintertouched feat, they expect a Wizard or Warlock to have only ice powers. Or if they ever design a +2 to damage against prone targets feat, that they will assume that every party member will have powers that knock enemies prone.
Maybe the system itself is already set up in a way to avoid the worst, by virtue of using powers, not permanently available abilities, to do the "crazy" stuff.