I thought about it during his development. I thought a lot about all the champions and regions!
If Sylas was born there he'd probably be Swain's right hand man - an assassin/rogue like Katarina perhaps. Being able to trace magic helps you not only hunt targets but spot magical traps, see through illusions and circumvent protection. It's sad really. You are right also that a powerful nation that embraces magic would make a useful ally for a band of rogue mages unwittingly camping on a secret vault of worldstones.
Alas being in a dungeon for his entire adult life Sylas probably knows very little of Noxus (and I believe the factions encountered each other late enough there might not be many records in old books he had access to), and would be immediately wary of any empire and their rhetoric. He's not exactly the trusting type. Swain would be all like 'In Noxus, strength is what decides who leads!' and Sylas would say 'And yet here I see at the top of the pile, a cripple who had to borrow his power...'
Sylas' line about Noxus likely comes from a place or ignorance or at least indifference. I know it makes it confusing but I don't think you can always trust that our champions are speaking truthfully. Or with wisdom! Except Zoe. She has it all figured out.