I have a simple gps watch that uses my weight to estimate calories and doesn't measure heart rate. It gives me the same as you, just under 100kcal/km for the same weight. But I'm convinced this is a dramatic overestimation, I run a lot and have for years, and I'd be eating way more if I really burned that much. I think with regular exercise we become extremely efficient and the calorie burn goes way down. I don't know if heart rate accounts for that or not, but overall I'm sceptical about calorie counts.

When playing beat saber, I always assumed the Apple Watch over-estimates calorie burn if only because the watch is on the part of your body that's the most mobile (your wrist). So, getting 300kcal for a 30 minute session felt like cheating.

But, there's at least "more legit than an Apple Watch" evidence that the calorie burn is that high; 6-8kcal per minute on Expert difficult [1], and some of the Expert+ tracks could easily burn more than that (and, I weigh much more than their Standard Reference Person).

I'm not the most fit person, but I've seen surprisingly little hard evidence that the Apple Watch grossly under or over-estimates calories burned. I imagine it starts getting worse at the extremes, and nothing is perfect, and everyone is different; its an area where more research would have unbelievably high leverage (Apple has sold, hundreds, of millions, of watches) (in fact: I'm sure they've done some of this research themselves).

[1] http://vrhealth.institute/portfolio/beat-saber/