The problem with Molyneux is that his logic sucks. UPB and Molyneux just miss the mark entirely. It’s frustrating to try and read UPB and I would not impose that upon anyone.
That said, UPB and estoppel are not the same. UPB is about trying to create some sort of universal ethic, as if everyone ought to accept it and live by it. Estoppel is not about that. There are a certain class of people who are against estoppel (read hypocrites). They have a vested interest in the law being inequitable. So while Molyneux might claim that estoppel is somehow UPB so long as people value justice or whatever, he would be misunderstanding the purpose of estoppel. I have no idea if Molyneux makes this claim, but it seems like the sort of claim he would make.
The reason why estoppel was adopted as a legal principle is because when there is a balance of power in society, people prefer to be treated equally. It is when an individual or a group of individuals gain power over others that estoppel ceases to be a principle. Clayton didn’t really explain estoppel in A Praxeological Account of Law, but he does explain the difference between a society with the bully and a society with a balance of power.
In short, estoppel is not UPB. It exists as a principle precisely because there are hypocrites.