Largely just semantics in everyday usage. The technical distinction is that idrott emphasises physical prowess, whereas a sport may not necessarily do so. For instance, some consider chess a sport, whereas nobody would think it's an idrott. On the other end, there are also people who don't consider e.g. equestrian events or motorsports idrotter, although this is (obviously) more debated than chess.
What is ''Nordic walking"? Back packing? Hiking from hut to hut in Norway?
Ha Ha! Walking with ski poles for a full body exercise. I thought this is just what we old people do so we don't fall over when we hike! That's why I use treking poles!
Can't speak for the Swedish preceptions, but being good at many computer games or video games typically requires good reflexes and good physical dexterity (and in some cases some degree of endurance, many computer games that are played as sports actually have longer matches than some traditional sports and usually require attentiveness and maintaining those reflexes and dexterity throughout the entirety of the match), so they're not as purely mentally focused like, say, chess or many board games (and yes, there are people who treat some board games as sports).
I find myself wondering why a translation of "Golf is a strange game" is not accepted. In at least American usage, games don't refer simply to idle ways for children to pass their time. A question of "Did you see the game last night?" would almost certainly refer to (American) football, baseball, or basketball, depending on the season of the year.
How common is to say idrott? 'Sport' is international and most common word so I'm just wondering is idrott something Swedes often say when they talk about sport.
Totally unrelated to Swedish, but it reminded me of this Supreme Court case: https://en.wikipedia.org/wiki/PGA_Tour,_Inc._v._Martin
Which I found out about listening to a Michael Sandel lecture, and the discussion about whether golf is a sport or a game: https://www.dailymotion.com/video/xgih85 (from minute 4:57).
Until I saw that video (I was actually watching Sandel's "Justice" course on YouTube, which I highly recommend), I didn't know so much philosophical discussion could be derived from golf! And then I found this thread XD