But because after accounting for human inefficiency, for cycling purposes we can work from 1kJ = 1Kcal.

Strava uses a 1.1Kcal = 1kJ calculation if you have a PM, and I use that one because I have power2max meters which report notoriously low.

I have never gotten any ridiculously high calorie burn reports from Strava, even pre-PM. I seem to recall some ~30% high (by my post-PM math) but nothing like the people seeing double or triple Kcal to kJ.

Edit: That calorie calculator on the Bicycling Magazine linked above is just bonkers. I put in my numbers from my 64 miler yesterday, and it gave me a result of 4,182Kcal. Man, in my dreams. That ride was 2,135kJ via my PM, or 2,380Kcal according to Strava.

Bottom line: 900-1,000Kcal/hr takes a pretty much wide-open effort. My FTP tests are around 1,000kCal/hr. During rides, I ballpark-guess based on 35Kcal/mi. 64 miles X 35Kcal = 2,240... which is pretty much spot between the PM and Strava.