No, they didn't. UB is a cop out and inserting yield is just plain bad. Locking up one or more threads in an implementation defined manner would be the outcome of least surprise (I already know it's going to lock up at least the one thread).

The standards intended interpretation of UB was always intended to be something like "implementation defined, no documentation required" to allow for implementation weirdness, even unpredictable ones. It was compiler authors who decided do abuse this allowance to do really unintuitive things instead of weird platform weirdness.