I don't think the poor HTML rendering and extra Microsoft code was what laurensr meant when they mentioned the lost features in the new Chromium-based edge; rather, it's the loss of Edge-only program features like pen/ink markup on pages. https://support.microsoft.com/en-us/help/4027048/microsoft-e...

They're not interested in bringing the inking features over? Did they fire the team or something? Is there something about their code which makes it not worth porting on to Chromium or something?

All the inking stuff (and also the PDF/EPUB stuff that old Edge was really good at) is in UWP (WinRT/XAML) controls. At the moment it looks like the Edgmium team is entirely avoiding UWP controls. It seems like they are avoiding them for maximum cross-platform ease, but it's still not hard to wonder why they don't just use XAML Islands for nice stuff like Inking on Windows 10+ at least.