the negative -2 speed is applying on friendly large units also. to repro it get a cyclopalypse cannon. (or any other large unit) place mammoth club on the cannon. and you'll get the -2 speed penalty even though the unit is large.
update : mammoth club also has two other bugs it seems. The first bug makes the negative 2 speed penalty permanent. so say you shatter the mammoth club the debuff won't go away. so in theory you could shatter the club place another one on the same unit and even if they shatter it they'll have -4 speed for the rest of the game. unless their faction or split deck has access to redeployment effects. i don't believe this is how mammoth club worked before but maybe i could be wrong. as for the 2nd bug it seems large units aren't receiving crushing blow from the equipment. so basically due to all these myriad of bugs mammoth club as an equipment is completely unusable for the most part.