During a conference with Felicity and Diggle, Oliver admitted to his partners that he's seen people with enhanced strength and speed before--on the island, years ago. He explained that during his exile there, he came across a Japanese military sub that housed a serum meant to turn people into human weapons.
"Human weapons?!" Diggle responded incredulously. "What's next, aliens?"
That off-handed dismissal of aliens certainly sounded like he perceived them as not being real--as though the idea itself was a bit preposterous.
Which, really, doesn't seem like the kind of thing you'd say when you've just seen a batch of aliens take apart half of Metropolis over the summer.
While far from conclusive (after all, a minor tweak of intonation could be less incredulous and more exasperated--"What's next," as in they can't catch a break--in a world where "We haven't talked about that becomes widely reported as "it's not going to happen"), that line certainly felt to us like an in-continuity indication that aliens are still not a thing people believe in on Arrow, and therefore that the massive Kryptonian battle over Metropolis we saw in Man of Steel has not happened here.
Interestingly enough, Oliver brushes off the alien suggestion and moves on to continue explaining the serum with only the the comment that he'd seen some unbelievable things during his time on the island. Hmm...!