Hi, yes that's right, it's part of the controlled feature roll out. I have never even seen that new design until now in your screenshot :) would you mind mentioning which Edge insider channel and version are you using? also how long it's been available?
so yesterday I installed the latest Edge canary (Version 81.0.368.0) but that didn't change anything. today I cleared everything (website data, app data, cache, cookies etc) in my Edge because I was having some problem and then I noticed I have the new InPrivate design!
that made me think that Microsoft is actually controlling who gets what features using the cookies saved in user's device. I don't think Microsoft has an active online server that controls and tracks the controlled feature roll-out or A/B testing.
I will try this again in the future to be 100% sure. anyway thanks for sharing the news about it :)