I don't think the latter part is true. They were on the same team at the 5 Nations tournament, at times the same defensive pair, and I thought Boqvist was Sweden's best defenseman defensively. Lundkvist wasn't far behind, and he's no slouch defensively, but I think this narrative that Boqvist can't defend is very wrong.
I see the difference as Boqvist is just a lot more dynamic. Better skater, better shot, better passer, better stick handler. I would consider Lundkvist to be closer to Erik Brannstrom than Boqvist. A lesser version of Brannstrom, but potentially still a first round pick.