I was wondering why he couldn’t just use the normal way of modifying configuration files to configure Xfce.
Here on KDE, about half my configurations are stuff like configFile.kwinrc.Windows.FocusPolicy = "FocusFollowsMouse";, which basically just modifies ~/.config/kwinrc to contain a section:

[Windows]
FocusPolicy = FocusFollowsMouse

This isn’t always pretty, especially when the configuration format is itself not the prettiest, but it seemed a lot simpler than using this xfconf thing.

But well, the reason is that Xfce’s configuration format is severely ugly. Namely, it’s in XML.
XML is a markup language, not a configuration language, so it has a far more complex structure, which you cannot easily formulate in the Nix syntax.

You could still include those XML files as a whole and have Nix roll them out with some mild templating. But yeah, at that point, Nix doesn’t actually do much.

I don’t agree that you have to use Sway (configuring KDE works well for me, for example), but yeah, some pieces of software will just not have decent configuration files, so if you want to use those, you have to configure them the non-Nix way.

I do wish NixOS wasn’t as much of an investment, so that folks could just appreciate it when it helps them configure something and wouldn’t be disappointed where it can’t do that.
To some degree, I feel like using just Nix Home-Manager on another distro alleviates that, but it has some other pitfalls and I’m not yet wizard enough to know all the benefits that NixOS’ particular way of doing things brings along.