Didn't you write somewhere aerial creatures already implicitly have flying?

Yes, however I think there's still value in having some evasion in the set. I tweaked to non-aerial creatures for this evasion, because black usually doesn't get "cannot be blocked" outright. Since swinging for discard would probably be blocked as often as possible, I needed to include some kind of evasion.

But why give evasion that is flying to a creature that has flying? Menace is evasion. "Can't be blocked by creatures with mana value 2 or less" is evasion.

I forgot Menace existed for a minute. Is "Can't be blocked by creatures with mana value 2 or less" something that is currently used? I've seen green get evasion from creatures less than two power. Evasion of mana value two or less didn't give me anything when I checked Gatherer (admittedly, I searched the whole phrase and I might've needed to break it up for better results). I'm not opposed to that, but it gives me vibes too close to green's power 2 unless it already exist.

I just realized that you meant the mechanic flying when asked if aerial creatures implicitly have flying. Flavorfully, aerial creatures implicitly have flying, but there no mechanical relation or connection between the aerial supertype and the ability flying.

No, I don't think there's evasion based on mana value, other than a handful of cards with protection. I think SI was referring to daunt, green's "power 2 or less" evasion