Title is slightly misleading. According to the article, it’s the ability to not scar, but instead grow new tissue, that’s what is switched off in mammals. This isn’t regenerating lost limbs, it’s growing tissue— and the tissue it grows at the amputation site isn’t correct at all, which sounds like a good reason for evolution to switch it off.
It’s still exciting research, because they hope to figure out a way to get stem cells to regrow the correct limb at the injury site instead. But that’s just a hope right now, and very speculative.