and since I determine what is PI...
No, you don't. The license defines Product Identity. If you
declare something as Product Identity that doesn't meet the definition provided by the license, your
declaration is of the same legal effect as a declaration that you are the Emperor of the United States.
The definition of "Open Game Content" does include an additional "any additional content clearly identified as Open Game Content by the Contributor" provision. So stuff that would otherwise not meet the license's definition of OGC can be made OGC by declaration of the copyright/trademark owner.
The seemingly parallel "clearly identified" provision in the definition of "Product Identity", on the other hand,
limits PI to stuff that not only meets the rest of the definition, but is also clearly identified as PI. So stuff that would be PI can be made not-PI by failing to declare it, but stuff that otherwise doesn't meet the definition cannot be made PI by declaration.
So, OGC is anything that meets the license's definition of OGC
or that has been declared OGC by its owner. PI is only that content which both meets the license's definition of PI
and is declared PI by its owner. This is a deliberate, intentional bias in the OGL to having things count as OGC, and not count as PI.
Now, will a random Federal judge, in the event, rule that a given monster stat block is the "game mechanic", or "derivative" of OGC, and thus OGC by the license definition? Nobody knows! There's hardly precedent on the definition of the term "game mechanic", and what constitutes a derivative work is a matter of vague guidelines.
So, by declaring the monster stat block PI, you have taken that uncertainty about how a judge would rule on the meaning of the OGL in a specific case, raised it as a banner, and declared "Come and try me!" Since your monster stat block probably isn't worth the hassle of a fight, you may well get away with it. But the effect of the uncertainty and hassle of the legal system is hardly the same thing as the OGL giving you
carte blanche to "determine what is PI".