Thanks for the additional perspective.
We have 70+ packages that expect the <if @anyvar_p@> or <if @anyvar_p@> to evaluate true for any non-zero integer. In the short-term we will patch OpenACS 5.9 back to the 5.8 behavior. If the community has a compelling need to have integers other than 1 return false, we will probably eventually update our packages to work with the new tag behavior to minimize the number of core customizations we have to maintain across updates.
We will submit a bug and patch.