Say a foreign law enforcement entity is investigating Mr X, and asking a domestic authority for some information on Mr X.
The treaty says that generally speaking, the domestic authority should provide such assistance.
However, assume that instead another domestic law enforcement entity was asking the domestic authority for information on Mr X, but (under purely domestic jurisdiction) the domestic authority would be prohibited to provide such assistance for some reason (say, due to privacy laws, procedural protections, or so).
Then, the foreign law enforcement entity would not be entitled to the assistance, either.
You kind of just reworded what I wrote but I don't see how it changes any of the concerns I expressed.