The aluminium on those Magura brakes doesn't break from normal use. Clearly, it's been bashed at some time. The questions are by whom, how and when?
If the part weren't strong enough, that failure would be all over the internet like Kalkhoff broken motors, but it isn't, which means that it's something special that happened to your bike.
If it broke on the first day, you could reasonably claim that it had been bashed before you got it, but if I was the judge in your case with the above evidence in front of me, I think I'd be pointing my finger in your direction. Sorry, I'm still empathetic with your grief, but I don't think you can blame Haibike or the dealer for this one.
You have one get-out. If you look at the broken surface and it's uniform crystaline appearance, it was made properly, but if there's any sign of an inclusion or void that caused it to crack or break, it's down to Magura.