Yes.
But why? Debian is probably updated more actively and that’s probably the reason why so many distros are based on Debian. Also, most Purism developers are experienced Debian developers, so it’s a natural (and efficient) choice.
Just because you have some extra packages, doesn’t mean you have to do all the branding differently.
“What OS do you have on your box?”
“Debian!”
“Vanilla Debian or Purism Debian?” ← - - That question you’ll never hear
“Purism Debian of course. I want the privacy extensions, but not the binary blobs”
“How do I install it?”
“Oh, simple. Install vanilla Debian, remove some extensions, add Purism repo, install 50 Purism packages. Shouldn’t take you more than an hour if you’ve done it a few times. One time I got a dependency conflict but usually it’s fine”
“Uh… why didn’t they just make a derivative like everybody else?”
“I think they wanted to save the hassle of exchanging some logos”
“Seems legit”