Would be a sound argument if your basis wasn't completely wrong.
A "pure" deadlift is breaking the weight off level ground (no deficit or blocks), and locking out without hitching or ramping the weight. How you perform that is up to you.
A lot of powerlifters pull with a rounded back because it increases leverage and their efficiency. And proper technique is all about leverage.
Yes, a rounded back a lot of times means your are way out of position and that you are in danger of hurting yourself, but use it correctly and someone may put themselves in a more efficient position. Basically shitty deadlift form usually comes with a rounded back but a rounded back deadlift isn't always an indication of bad form or what you call a "pure DL" lol
If you got a problem with that, you should go tell KK, Pete Rubish, Chris Hickson, and a bunch of other top level guys that they're deadlifting wrong and show them how to deadlift. Hell, you can show ME.
I think you misunderstood when I said rounded back (probably cause I didn't specify). I mean a back that rounds as you're pulling. IE: Abs not braced and locking your starting back position in whatever state you choose.
Like in the video from a couple pages ago where, as the guy is pulling he goes from taut to rounding.