Well yes but those things are part of the startup checklist. Every plane has that, even on a Cessna 172 that does have a key (that doubles as ignition/magneto switch) you can't just turn the key and expect it to fire up. At least not if the power-down checklist was followed properly and the mixture pulled to cutoff! If the engine is cold you'll also have to prime it first. I'm sure on an F-16 this is a magnitude of times more difficult but it's not really a 'key' as such, it's just process. Once you know it it can't be taken away like a key can, or changed like a PIN code.
They're not protection against unauthorised use. It's more a side-effect. I mean more like a key that's different for every plane, or a PIN code or something. But maybe if there was that'd be confidential information I guess, maybe I shouldn't have asked :D
PS: I didn't know the new F-15s were so different. I'm really surprised they put them back into production. I remember them roaring over the Netherlands from Soesterberg but this feels like ages ago.