This makes complete sense! Jago would feel absolutely no compunction about lying to Bren if it made things more convenient, about Tano's status or anything else -- but him being a not-yet-qualified assassin fits very well.

Are we told at some point that Tano is quite young-looking? It might just be the way I picture him, round-faced in contrast to Algini's angularity; I don't know if it's in the books somewhere or just what I made up.