Correct, however, the MTR is the unit that is using a FIRE ATTACK A 7. As Spotting is listed in "any other action" instead of in Case B "...fires or directs fire, …." obviously the spotter isn't firing or his EXC would be in this section. Instead since the SPOTTER isn't firing, he gets placed into "any other action" Case C.
And he is considered to be using a SW for "MOVEMENT and IFP loss restrictions" only it is NOT a fire attack. Opp fire is a Fire attack...you simply mark the spotter to designate that a spotter is being used and the appropriate restrictions apply.
If the unit also has a SW he WISHES to FIRE, ie. use the A7 rule section, then his concealment loss is governed by either Case B or D as appropriate...its straightforward.