Estoppel is a long-established legal doctrine (centuries old, in fact). One party cannot mislead another to its detriment through promises expressly made and later retracted. In such cases, the courts will use their equitable powers (i.e., doctrines such as that of promissory estoppel) to ensure that an unjust result does not occur, even if the party might otherwise have the legal right to enforce its claim.
The devil here lies in the details. The Microsoft Community Promise is indeed very explicit and says that it is. In effect, it says, "don’t rely on anything other than a literal reading of our promise." And it specifically says that no rights of Microsoft are being waived by estoppel, among other things.
Therefore, unless someone is covered by the literal terms of its promise, Microsoft has reserved lines of attack for itself should it choose to exercise them.
Estoppel might still work but it would take a costly litigation to determine the outcome. Who wants to put themselves through that?