I think you have a some what confused understanding of what the various roles and duties of these government bodies are. Whatever the purpose of the Treasury Board is (which you do not seem to fully understand since they are not a corporate treasurer), it has nothing to do with whether the CRA operates as a corporation. Furthermore, the CRA is not subservient to any other federal department as it is its own department chaired by the Minister of National Revenue.
Canada does have government bodies that are corporations, as I said the Bank of Canada is one of them, Canada Post is another. The full list can be found here:
https://www.canada.ca/en/treasury-board-secretariat/services...
The CRA is no where on that list.
Here's a bit of a challenge for you to help you clarify your understanding of this. Find one single semi-authoritative source that refers to the CRA as a corporation or describes the operation of the CRA as being like that of a corporation. I'll even take an informal source.
You won't find any, since no one describes the CRA as a corporation and doing so is just confusing. You made it up, and given that Canada does have bodies that operate as corporations, you shouldn't be surprised that making something up like this will elicit disagreement.