I have checked that 1-hour on the treadmill makes a 10k plus activities with a pace of 6 km/h. So, if someone has a job where he needs to walk a lot, 50k activities is not impossible (a 5 hours walk approximately). And who are performing such activities on a regular basis (according to the leaderboard) for a long time, I think its genuine.
Thanks for the reply 👍 😀 🙂.