He's top-20 offensively as a C.
Hjalmarsson is 2nd on the Hawks in ES TOI, behind only Keith.
Seabrook plays a whole minute less per game at ES.
He's no #4 defenseman. He's at worst a #3 who may be ready for a #2 role alongside a true #1.
I was trying to say of all 2nd line centers, he's top 10 offensively, but you just bettered my point.
Hjalmarsson being 2nd in ES ice time doesn't mean he's their #2 defenseman, it means he can't play a lick of offense to see any sort of PP time. If you want to argue than Seabrook is Chicago's #3 defenseman, that's a battle you'll lose 10 times out of 10.
Maybe he's a #3 d-man instead of a #4 d-man, but a #3 defensive defenseman that can't hit is still not even close to getting a top 20 offensive center. If he was a puck-moving defenseman that's a different story.