Possibly a glitch or a hoax - Players reporting this Mythical reward!
(iMore iPhone)

Pokémon HOME players who`ve completed their National Pokédex are reporting a reward of Mythical proportions! What you need to know Pokémon HOME, which launched February 11, 2020, allows players to store Pokémon from across all the core games in with the cloud-based service. Pokémon HOME can store virtually all Pokémon from previous games, even those unavailble in Pokémon Sword and Shield. Using the National Pokédex system, players who collect every single Pokémon are reporting receiving the Mythical Magearna as a reward. Rumors are running wild about a reward some players are receiving for completing their National Pokédex on the new cloud-based service, Pokémon HOME. Reddit user, Scootio reports having received Magearna at level ten with an unusual amount of ribbons. Another Reddit user, DSDark11 reported in a post that has since been removed having not only received Magearna for completing their National Pokédex, but that their Magearna was in a special, previously...