suboptimal=/=bad Is it THE most effective way to grow your back? No. Can you still get a massive back doing deadlifts if you train hard and consistently? Absolutely. Should you do deadlifts for your back? If you want to. We gotta stop caring so much about what other people choose to do. Deadlift if you want, keenan flap if you want, idc.
Because it’s fundamentally a hamstring and glute lift. Sure your spinal erectors, and last will have to be involved. However if you go into it with a focus on your back you’re just risking pain and damage to your spine. You lower back was not made to pull such high weights. In fact as a bipedal evolution of primates. Our spines are pretty fragile.