You are making an big assumption there (i.e. Cassandra serving the Divine).
Varric is traveling with Cassandra, that is known since the GI reveal last year. They allied over a "common cause" after DA2 ending (we don't know what that is) and you really have to wonder why Varric run after Cassandra after the interrogation too...
As for not!Zathrian, he's the totally opposite of Vivienne in term of appearance. I can't decide if he is an apostate or a Dalish. His clothing are quite "outdoorsy", but they do not look Dalish one bit.
I think it's a logical assumption though, unless something major happens involving the Divine, Cassandra is still loyal to her.
She works closely with Leliana, who is close to the Divine, then there's the plot to Dawn of the Seeker (which I saw a while back, so my recollection could be hazy), and the fact she was interrogating Varric for her "precious Chantry" as he so aptly put it.
Still, I guess she may not be, but with the info (or lack of) we have now, I think it's safe to assume.
Sorry for going off topic

But I agree, he doesn't look Dalish.