If you re-read chapters when they are meeting Macht, yeah they fought 6 centuries ago. And also, one of the reason she stayed in the forest is probably she got PTSD after fighting Macht. Having your arm turned to gold for 1 century is no easy thing.
Nope it is still in character for Frieren. It is a win-win scenario for Frieren, not only she get random spells she wanted, she could honor Himmel's memories in her own way.