Summarising your question a lot, you are asking Does the C++ standard allow a compiler to assume a bool can only have an internal numerical representation of '0' or '1' and use it in such a way?
The standard says nothing about the internal representation of a bool. It only defines what happens when casting a bool to an int (or vice versa). Mostly, because of these integral conversions (and the fact that people rely rather heavily on them), the compiler will use 0 and 1, but it doesn't have to (although it has to respect the constraints of any lower level ABI it uses).
So, the compiler, when it sees a bool is entitled to consider that said bool contains either of the 'true' or 'false' bit patterns and do anything it feels like. So if the values for true and false are 1 and 0, respectively, the compiler is indeed allowed to optimise strlen to 5 - <boolean value>. Other fun behaviours are possible!
As gets repeatedly stated here, undefined behaviour has undefined results. Including but not limited to
- Your code working as you expected it to
- Your code failing at random times
- Your code not being run at all.
See What every programmer should know about undefined behavior
