Would the Gith trust us? Probably not. But we should have the option to tell him. The situation is dangerous and they cannot afford to completely ignore clues, even if they are suspicious of the person they got them from.
As the

Bhaalspawn slayer

is in the game files there will be an appearance. Bhaal is still around and involved with the tadpoles. What if the tadpoled absolute followers turn into illithid slayers under total mind control by the absolute.

I sometimes use thought experiments. I don't necessarily believe in every idea I post for discussion on this forum