@Pierre Did a quick check with a handful of rides and compared the Whoop data with the values generated by my Wahoo Elemnt (with a powermeter).
I checked 7 recent rides with energy output between 377 kcal and 5.365 kcal (no typo). Whoop tends to underestimate kcals ~10%, but one ride was overestimated with a 46% larger output compared to the Wahoo, and one ride was underestimated with a difference of 33%.
Depending on the formula Whoop is using to derive calorie usage from HR, it might make sense that it tends to underestimate actual output for a reasonably trained cyclist.