If you mean with that that the Speed stat gets higher when you use Hammer Arm while being in Trick Room then that's wrong!^^ The stat still gets lowered when you use Hammer Arm: But exactly that is helpful in a Trick Room: Because now, every lowered Speed stat will be faster than any increased one!^^ So if your opponent uses Agility and you use Trick Room, the enemy's speed stat is higher but this also means that in the Trick Room, the Pokemon is very likely to attack last!^^
Or, to kepp it short: Trick Room does not affect stat changes!^^ It only affects the effects of those stat changes (meaning that being slow is a good thing and being fast is a bad thing in Trick Room)!^^