People are confusing "nickle cornerback" with "slot cornerback."
A nickle CB is a fifth DB who enters the game in nickle packages.
He can play outside or in the slot (example: when healthy Jimmie Ward plays outside in base and in the slot in nickle, He therefore is not the 49ers "nickle cb" -- Rashard Robinson is the nickle CB even though when he comes in on nickle downs he plays on the outside).
Put another way, unless TM is entering the game for nickle packages by definition he is not the Cardinals' nickle CB, regardless of if he lines up in the slot or not.
It's also kinda bizarre to refer to TM as the Cardinals' nickle CB because in their hybrid scheme they're regularly rolling out there with seven DBs, not five (five == "nickle").
Likewise, when they're rolling out there for short yardage in a 7-3-1 TM is playing OLB or (more regularly) the single safety, and if they're in 5-4-2 short yardage he's playing WLB or outside CB.
If you think that of all the positions he lines up in he's best in the slot that's fine, but it's not on the topic. If you want to argue he spends most of his time in the slot or in a "dummy slot" role that's also fine, but it still doesn't make him the Cardinals' nickle CB.