The humorous response from the “Dungeons and Dragons: Honor Among Thieves” team is tongue-in-cheek, but it’s a reasonable response to fan criticism. The dungeon master in the YouTube clip is invoking an ancient “D and D” precept called the “rule of cool,” which states that bending the rules is okay if the result is sufficiently badass. Allowing a druid to transform into an owlbear certainly meets those criteria.
However, fans are correct that the transformation is not technically allowed. Druids generally rely on the “wild shape” spell to take on the physical appearance of an animal, but the rules for that spell only allow the caster to become a beast. An owlbear is an amalgamation of bear and owl, and is therefore technically not a beast. Instead, the game categorizes it as a monstrosity, since beasts must be naturally occurring in-universe. There is a high level spell called “shapechange” that would technically make the owlbear transmogrification possible, as it allows the caster to become anything other than an undead or a construct, but it’s unclear whether Doric has reached that level in the “Dungeons and Dragons” movie.
But again, as any seasoned player knows, each dungeon master picks and chooses the rules their table follows. It’s part of what keeps each campaign unique. If “Dungeons and Dragons: Honor Among Thieves” wants a druid to become an owlbear, you can either accept it or find a different game.