DetSys founder here :). Firstly, congratulations to Flox on the launch.
DetSys is founded by Nix's inventor Eelco, and me -- a fairly long time community member (since ~2016.) We're also loaded with long term Nix users and contributors. We (the company) didn't invent flakes, and I didn't have anything to do with it either. While working at Tweag, Eelco worked with Target to develop the idea.
I'm not sure where this idea of bad blood or not contributing back comes from, either. The vast, vast majority of our code and products are completely FOSS, and our most popular product (the Determinate Nix Installer) is deliberately licensed in a way that upstream Nix can take it and integrate it however they want, any time they want. And I hope they do! We also invest heavily in contributions directly upstream. We're long time open source contributors and advocates, and that doesn't change now that we started a company.
I think there is a lot of compression of time and history here that has lead to lot of pretty baseless accusations and confusion. I think some people who don't like the change don't like it because they've built their own mechanisms to get what flakes provides. That's fine, nobody has to use flakes. The old ways aren't going away. Some folks don't like them because they're not technically perfect or bug-free. That's fine, too, but they're not becoming more stable by pretending flakes don't exist.
I look at threads like the one you linked as evidence we're doing something interesting. Nobody would care if what we were doing was useless.
One point that is absolutely true, though: DetSys is 100% contributed to the version of the future where Nix + Flakes is the thing. Flakes solve such vast problems that I feel there is no going back. This is the basis for the Determinate Nix Installer: we guarantee flake stability, and use the installer as the wedge to keep that promise with our users. This promise has been the basis of our work on FlakeHub and our just-launched-today FlakeHub Cache.
Unfortunately, nix.dev -- an official Nix documentation source -- specifically does not teach much about Flakes. To the degree of introducing documentation for unmaintained tools that see very low community adoption: https://nix.dev/guides/recipes/dependency-management.
We've taken a strong position on flakes by introducing our installer and zero-to-nix.com, a flake-first documentation source because of this confusion. I have asked numerous times for the various Nix leadership teams to make a choice on if flakes are the future or not, but as we can all plainly see: they have not chosen to do so. This confusion about flakes comes from the very top, unfortunately. I find this very motivating for building the best Nix company I can, to support the adoption of Nix and flakes in interesting places around the world.