Guess what Edge and Chrome do if you make Firefox the default browser? They all harass you if you run them when they aren’t the default. At least Firefox respects the “don’t bother me about this again” checkbox

I think “complex” refers to the various dark patterns used by Windows and Mac/iOS to scare and/or force users that know nothing of computers into using the default browsers.

I think the article is talking about prompting users to choose a default browser without necessarily installing them, during the OS first run for example.