I'm not sure what spell of effect the npc was using to summon shadows, but the impression I got was that it wasn't 1 shadow per round but perhaps a handful of shadows all at once, and I totally buy the notion that they're very dangerous in numbers, even against a high-level party.
After all, the shadows' strength drain attack reduces fighter-types' ability to, well, fight. That strength drain is bad news, especially if the party's healer has to keep burning spell slots to prevent the fighter from dying from strength drain while they're under attack by something that's damaging hps, too.
Regardless, the guy who was in the combat says it was a tough combat. I don't see any reason not to take him at his word on that.