When syncing a power meter ride from Garmin Connect to Strava, Calories shown are identical for me (or off by less than 1%, usually because of the way strava uses moving time instead of total.)
Comparing this with non-PM rides from GC, the amount of Calories in Strava seems to be overestimated by 10-20%, especially for low intensity rides. So it’s still a reasonable number if you are looking for a ballpark figure. If you’re doing calorie counting/MFP on the other hand, it could be enough to kill your deficit for the day 