The same could be said about 1920 agreement but "somehow" for it it was perfectly valid.
Really? Perhaps you can provide us with some details about the ultimatum which Poland was given in 1920? Who gave is and what threat backed it? I mean, we all know about the ultimatums which interbellum Poland handed out to countries such as Czechoslovakia and Lithuania (i.e. do what we tell you or we'll invade you) but this is the first we have heard about Poland being given an ultimatum in 1920.
Or am I wrong saying that Britain and France signed the Munich Pact with Adolf the Great volontarily and without any pressure from external circumstances?
Shock, horror: Britain tells one foreign country that it can do what it wants with regard to a border dispute and tells another foreign country that Britain isn't going to help it with regard to that border dispute. Or perhaps you can tell us about the treaty which Britain had signed with Czechoslovakia in which support was promised?