Yeah, up until 3.x Paint.NET was released under a MIT license in a source available sense (no outside contributions accepted, just source dumps with releases). The author didn't like when people would relabel paint.net and release it (sometimes for a fee), so he stopped doing that somewhere during 3.x.

Pinta is based on using the engine for one of those with a new Gtk# UI. Though at this stage a lot of that's been rewritten, so it's pretty much just some filters that are used as-is from old Paint.NET.

Even though Paint.NET moves much slower these days, Pinta has moved even slower and the UI rewrite was never as polished as Paint.NET's WPF UI.