A valid argument.
I suppose a lot depends on how the Typhon Pact is setup. If it is indeed like NATO (which is a military alliance ala that between UFP and Klingons) with the individual powers retaining their political individuality in terms of their foreign policy, then I suppose the treaty would have to be honored. If however, the Pact is like the UFP (ie. single political entity with a single foreign policy), then any treaty made by formerly independent members with neighbors wouldn't carry much weight and I think likely to be discarded.
Well, A Singular Destiny
seemed to make it clear that the Typhon Pact is more of a very close-knit alliance than a sovereign state like the Federation. I'd say the closest equivalent would be the European Union, actually -- delegated sovereignty in some areas, divergent in others.
But.... Even if the Typhon Pact were
a sovereign state, that does not mean that the treaties that its members made would be rendered invalid. In real life, international law holds that successor states are still party to treaties made by their predecessor states. Thus, for instance, the Russian Federation is still party to the treaties on nuclear reduction signed by the Union of Soviet Socialist Republics. Or, for an older example, the United Kingdom of Great Britain and Ireland and United States of America under the U.S. Constitution in 1801 were still party to the 1783 Treaty of Paris between the Kingdom of Great Britain and the United States of America under the Articles of Confederation.
( an analogy would be if California had had a treaty with Russia, it would have been voided once California had become part of the US)
Actually, the United States would probably have been forced to assume California's obligations under the treaty. (Of course, bear in mind that the legality of the so-called California Republic's independence from Mexico is questionable, since it was proclaimed by European-descended Americans who had moved into the Mexican territory of California in order to capture it for the U.S. from Mexico.)
I doubt if any treaty the vulcans or andorians had with the romulans and klingons continued after they created and joined the alliance that would become the UFP.
Well, we do
know of at least one treaty between the Federation and one of its Member States that was continued with the UFP as party after the first Federation Day: The treaty that ended the Earth-Romulan War. It established the Neutral Zone and provided that breach of the Neutral Zone by either party, the Romulan Star Empire or United Earth, would constitute a declaration of war. The Federation is established to have become party to that treaty in "Balance of Terror," even though it did not even legally exist when the war ended and the Neutral Zone was established.
More than likely, the UFP continued the Vulcan, Andorian, Tellarite, Alpha Centauri, and United Earth obligations of any treaties they made prior to the signing of the Articles of the Federation.