That shouldn't matter. They might even have a Linux version in the works, given they are doing Windows now. They are within their rights to ask you to get rid of it. You also seem to be using their brand assets like the logo. I get your intention and love what you were able to accomplish but if you intend for this to grow, you are better off doing this now.
True, but a cease & desist will likely include the demand that they cease all operations, and they might be willing to fight that in court, possibly seek damages, etc however likely or unlikely it is that it would hold up. They can pay for a few hours of a lawyer's time to intimidate the OP.
The launcher UI is AppKit while the extension APIs allow third-party developers to write them in TS and a React subset. They have a custom renderer for React that turns that into AppKit primatives. So similar in concept but custom and specific to their usecase.