Well, I checked Celebi's IVs on this... No possibilities.
However, I checked my own Brave TRU Mew's IVs and it had No possibilities. Then, I checked my own Sassy TRU Mew (I got 4 Mew from that event) and it had a possibility of Method 1 or 2. @_@
I.e. some GBA Event Pokemon do not appear to be generated like other Pokemon. Ask RNC for a legality check on Celebi? I actually do not have a flashcart.