Yes, the Prime Directive entails sheltering developing cultures (precisely: pre-warp cultures). But it is broader and also does protect developed cultures. Memory-Alpha describes it as a Non-Interference Principle and rightly so. This applies to all non-Federation cultures and factions that are, to sum it up, to be left alone.
One can interpret this directive as some sort of principle of non-intervention comparable to the principle with the same name of contemporary international law. In this case, it would indeed not be applicable to interstellar treaties as it would focus on internal affairs that the Federation does not hold any (immediate) stakes in. Interstellar treaties would, in contrast, be considered external affairs as the Federation is a Party.
But one could argue that even in case of a non-intervention principle, the Prime Directive could oblige the Federation not to interfere in treaties in between foreign cultures. One could, e.g., argue that the Federation, interfering in a trade-agreement between the Bajorans and the Ferengi, would be in breach of the Prime Directive as it interfered in the affairs of others.
Just one small step further, one could interpret this directive in the meaning of a general principle of the rule of law. In this case, it would make no difference between internal and external affairs of other culture; the Federation would have to respect both. It would just say: Respect the laws that you encounter.
I, personally, would, for my games, take the position that respecting treaties with foreign powers would be a matter of Starfleet Regulations, but not necessarily the Prime Directive. But I see the other position has good arguments and would not argue that a broad interpretation in style of “Prime Directive = Rule of Law” was “wrong” of some sorts.
In my opinion, it depends on how the players see Starfleet, the Federation and their principles.