One didn't have the title of 'Illusion Celestial' without an absurd prowess in Illusion Manipulation to back it up.
If Belphont was a god, then he'd be the 'god of Illusions'. That's just how good the man was with them.
Even with his resistance to Illusions granted by Adaptive Evolution, Evan didn't notice anything different until Belphont returned to his real form of his own volition.
But more than that, Evan was worried about Laurene and Kuro. So, without hesitation, he decided to play the card that he hadn't wanted to play until the end.
'Leviah…head to Olake and assist Laurene and Kuro.'
[Huh? Didn't you say I was to sit this one out?]
'Not anymore. Plans have changed. Inform Lloris that the Celestials are aware of our plans, and then go.'
Evan didn't know if the FCDs were truly aware of all their plans, or not, but given how Belphont seemed to be expecting him, he was going to assume that they were.