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. ( 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)
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.