It's suspicious, there's nothing that definitely gives it away. Cloning however is a pain in the Arceus if you don't have a Missingno, and if you went through the effort to clone it otherwise just to give away on the GTS...it was probably just hacked it.
Honestly, the remaining IVs are somewhat important. If it's all 31s and 30s, then you know it's hacked. They just kept the 30s in to get the Hidden Power of their choice (back when it still existed, Hidden Power is a move that changed types based on IVs). If the IVs are 30, then it's probably not good eugenics that got it its IVs.
Biggest indicator here is the name and met date. If the Swablu's name is in English and/or the met date doesn't exactly line-up (i.e. it's past 2017), then it's most likely obtained from the DNS exploit. Not technically hacked, but not considered legitimate in the circles that care about that.
I had a look at the site and I find it hilarious that in the FAQ section, they specifically avoided the word legitimate;
Italics mine. Basically, it's still very possible to hack a Raid and change IVs/shininess however you like and host that raid, so long as that species is usually available as a raid mon (that's basically all GF did in their crackdown, was to remove illegal species). But they did opt to not go for 31 in Atk, it's main offensive stat. Who knows, there's a small chance it could be real.
