In commemoration for the release of the TCG: Unified Minds expansion, an event Mewtwo was distributed to players in Europe via serial code.
Mewtwo was likely being distributed because it is one of the featured Pokémon for the new expansion.
Credits to Fade/seiferzero and Djoke12100 for helping us with this entry!
Format Ver.1.0.8-2, Post Updated Date:20190811_0349
Original news article: https://projectpokemon.org/home/forums/topic/54201-lgpe-unified-minds-mewtwo/