Yeah, events should have no nickname flag.
However, Ageto Celebi is difficult to check, as its Type is Unknown GBA, which is also used for Hatched GBA Pokemon, hacked GBA Pokemon, and very old events (e.g Channel Jirachi, Berry fix Zigzagoon, PCNY, Ageto). Celebi isn't hatched, so it's either the real event or hacked. That is not comforting in the least. I'm sure Secure has advanced methods to distinguish between the two, but whichever Celebi I had was a dud.