4th Edition Rules / 3rd Edition Scenarios

[Blackcloud6]

It's not redundant, it treats such dug-in units just like other units that are in limited terrain: when they fire, they get a spotted marker, and it remains on them as long as they are in LoS of an enemy unit.  The way Tread the Dinosaur wrote the rule, implies that in the turn after they have fired, they no longer can be spotted.   Actually, you can leave both versions of the bold text out, because if they are considered to be in limiting terrain, they will be marked with a spotted marker if they fire, and as you say, movement doesn't matter.