They do, if you don’t have 100% code coverage then you get what you get
100% code coverage isn't even close to a guarantee of correctness.
But is the easiest metric to guage test quality. What I’m saying is this code is probably not well tested
This code isn't tested at all. They messed up copying code from a previous implementation, that's how bad this is. If they had had a few basic tests in place they would've spotted this before it even had a chance to be a problem.
Then the problem has zero to do with the type of testing, now does it :)
Because the entire field is nonsense and pyramid schemes. Ethereum derives their smart contract langage from JavaScript for fuck’s sake. That alone should tell you that their interest was always to disregard everything in order to maximise adoption. And then they added pitfalls to JavaScript because it was not broken enough for their tastes.