True. But it is apt as an example, because of the strange nature of an open license. An open license is not like other contracts, whereby two parties agree to take on mutual obligations for a limited period of time. An open license, strangely enough now that I think about it, acts much like legislation; a right-holding entity publishes a document, generally available to the public and applying to the entire public, specifying how the public's rights interact with the entity's rights, and listing restrictions.
Basically, an open license is a unilateral contract (much like a "lost-dog-$500-if-found sign), offered to anyone willing to pick it up. If someone brings me my lost dog, I have to pay them. But if I take down the sign and post a notice retracting my offer, I no longer have to pay anyone who brings me my dog.
It would be different for a bilateral contract. If WotC had made a specific agreement with Pathfinder, they could not "revoke" their contract without breaching it. But bilateral contracts and unilateral contracts work differently.
The open license will always be valid for any products published while the open license was open. WotC can never take away Pathfinder, or Tome of Beasts, or anything like that, even if the license is revoked. But if the license is revoked, those companies could not publish any new material as of the date it was revoked.