Ok I just tossed it in and checked, and yeah - you do have to play the Adventure Mode to unlock tracks in the other mode. I've probably just never noticed this was the case because of course when I pop in DKR I go straight to beating Adventure Mode because it's great.
That said, in the Adventure Mode you literally can get straight into racing. Yes, there's boss races, and after beating a boss you need to beat their levels again and collect coins while doing so, but that's just extra challenge - race so well that slight detours for coins don't impact your ability to win.
I guess it can't be for everyone, to the point that some people even delude themselves into thinking the game controls poorly (lol), but I can at least understand where you're coming from even if to me it doesn't feel like a big deal (in fact it feels like a huge bonus).