https://wiki.guildwars2.com/wiki/Secret_Lair_of_the_Snowmen
Lists it as a strike mission and says: "Upon its introduction during A Very Merry Wintersday in 2018, the Secret Lair of the Snowmen was originally referred to as a group instance because strike missions had not been officially introduced into the game yet.[2] Since its return on A Very Merry Wintersday in 2019, it is labeled as a strike mission.[1]"