Yes, it is. If you suspect an individual of cheating, submit a ticket or PM a dev on the forum (@SaltyPatra and @Kafka are the most active). Otherwise, screenshots identifying other players in a negative light are considered callouts and are not allowed.
I agree that this is very suspicious, though, and the devs should be made aware.
Please see the Community Guidelines thread for additional information:
https://community.gemsofwar.com/t/community-guidelines/23732