He does not address JIT efficiency here. A likely implementation is just “lets drop all assumptions related to the type of A”. It would work, but frequent use would kill performance.

Python has all the issues that “become” has, with hundreds of “assumptions might have changed” points along common execution paths.

Liajit, for example, hoists everything it can outside of loops rather quickly, because it knows the cases “become” or Python equivalent can happen, and they are rare and uncommon in idiomatic Lua code.