I think it's a couple of things.
The NC State was into October, the first motnh of September is notorious for people doing outside stuff be it weddings, kids activities, wine tours, apples etc. Not saying it's a massive reason for lower attendance but unless its a hyped season like 2019 SU is going to usually struggle in Sept with the casual or fence sitting fan
The other issue is that for the ones who don't normally attend games this scnearior already played itself out last year for NC State when SU was as you noted 5-0 and it was a later game I think 3:30 as someone noted. An early season matchup with a 2-2 Clemson team despite the name recognition with an early kick isn't going to move the needle as much as it really should. If Clemson wins Sat I think that will help but I don't expect anything close to the NC state game level turnout.