Is it true that a scald from water boiled in a microwave is worse than water boiled in a kettle?
Yes it can be true. If you heat water very gently in a very clean container, you can actually get it to be above 100°C, maybe 105°C or 106°C.
And if the temperature of the water is 105°C, it will do you more damage more quickly. Also if you put in some sugar then it will boil quickly and explode all over you, and some people have been badly hurt by putting spoons into things and heating it in the microwave.