There are laws dictating how animals should be treated for research, and they don't specify at all how many animals may be used. They do dictate how research might be approached to minimize suffering and loss of life, and even with these weak animal welfare laws Musk still finds a way to run afoul of them.

The complaint relates to Musk pushing researchers so hard they make mistakes

its basically a fancy mouse. It lets you control a computer or phone. So, yes, as it goes inside the body I guess its technically a medical device but this is not a necessary evil. I fully agree with animal testing when it comes to important things such as curing diseases but this is just musk trying to roll out another device that will probably never work right anyways.

And killing a shitton of animals to do so.

I think the issue is more that they died from distress or complications of the hardware. Standard operating procedure is to dispose of animal subjects after testing irregardless, so the FDA isn't so much worried about the number of corpses as much as how they died.