What they are saying is that every language runtime, even stricter ones, has its own safety implications and you have to consider them when writing the specs an

What they are saying is that every language runtime, even stricter ones, has its own safety implications and you have to consider them when writing the specs and the code. Simply switching to a language with a stricter runtime may automagically take care of some simple cases, but no system is devoid of failure modes. Overflows are classic sources of disastrous bugs (e.g. the infamous THERAC-25). Switching to a system that detects them and raises an exception is a step in the right direction, but you still have to handle the exception cases correctly. And in some cases (like the Ariane V), incorrectly handling an exceptional condition turns out to be worse than allowing it to happen.