Well, I mean.
The Sun God has one fixed and stated every time characteristic: it's white. Even in this chapter it was highlighted:
"white clouds, white hair and white clothes". And it's not the first time the giants says that.
If anything, the Nika/destroyer - Nika/liberator thing is unsure, but those are objective characteristics: more so,
based on those Luffy, the real Nika, said "no wonder they think I am Nika".
As Luffy said, they already spotted the real Nika based on this.
So, Loki thinking he is Nika while not having any of those characteristics that everyone associate with Nika wouldn't be kind of dumb?