It’s illegal for them to do that, BTW. They have to prove magisk damaged your battery.
I ran into this with Dell when they tried to claim after market RAM was the reason a CPU core wasn’t responding to interrupt requests.
All it took was asking for the diagnostic data showing that the aftermarket RAM caused it to get the warranty repair approved.
You just gotta push back until they cave. Maybe ask for their mailing address for your FTC report or for the number to their legal department (most call centers are terrified of escalating anything to the actual company).
But, don’t directly threaten legal action, because they’ll stop the call right there.