It is far from true that a patentholder that knew about Theora for years would be simply unable to enforce their patent against Theora users.
It's not necessarily false either. The doctrine of laches (http://en.wikipedia.org/wiki/Laches_%28equity%29) says that you can't deliberately delay legal action in order to run up the alleged damages or prevent them from taking alternate paths. By publicly making potential patent holders aware of what they're doing, Xiph should be in a good position to claim that defense.
A laches defense in a patent suit generally only affects damages, limiting the plaintiff to damages based on infringement AFTER the suit was filed. If all the plaintiff is interested in is telling the defendant to stop infringing, laches isn't much of a problem.
It's not necessarily false either. The doctrine of laches (http://en.wikipedia.org/wiki/Laches_%28equity%29) says that you can't deliberately delay legal action in order to run up the alleged damages or prevent them from taking alternate paths. By publicly making potential patent holders aware of what they're doing, Xiph should be in a good position to claim that defense.
(In case it's not blatantly obvious, IANAL).