It's perfectly safe providing you're sensible.
Historically there have been some horrific instances where people have not been sensible and their babies have died. That goes for many things around the home and life though - stairs, cars, furniture, pets etc.
Thankfully heating milk up in the microwave can be done in such a way that it is no risk at all.
Firstly, you won't be warming it very much. We did it in 10 seconds increments initially. We warmed it to take the shell off and make it slightly warm, it's not in any way approaching hot let alone boiling. If you start off at 10 seconds and checking it, you're pretty safe. Remember that different microwaves have different power even the ones of the same wattage though.
Secondly you'll be swirling it or shaking it to dissipate any hot spots. Milk that isn't even tepid that has been swirled will be safe.
Thirdly you'd still tested it on your wrist.
The awful cases where babies died resulted from parents literally boiling the milk and giving it straight. In the same way that stairs can be a danger to a crawling baby but we use stairgates rather than insist on bungalows, microwave heating is only a danger if safety measures are not put in place.
The guidance is not there for a sensible parent but as a foolproof method to cover even the most foolish one.