Doesn't have to be Andii to hold a position in House Dark. Look at Draconus, for example. Elder Gods probably have no trouble taking the appearance of a Tiste anyway...and then think about the fact that a position via reading is not the same thing as position embodiment.
However, aside from the fact that Delat at some point wrote the poem 'Azath', indicating a very solid link with those structures and by extension their 'keeper' Gothos, there was a considerable amount of time between the formation of the BBs and the events of GotM in which h could have met Gothos (remember also Fiddler: "No-one knows Raraku like Quick Ben" or something like that...where is the Azath from DG located? Yeah...) without him being either Andii or Grizzin Farl.
What's more interesting is whether Gothos actually 'puzzled it out' or whether his "Well Gothos, sure" is more akin to him being more open and directly linked to the ancient Jaghut. And Rake...the closest we've got to Rake's knowledge is what Whiskeyjack told him, but this passage suggests the Son of Darkness knew more than what he was told. So there is definitely more to what is being talked about here than his multi-soul nature. Now, if SE has killed Quick off 'for good' in DoD and we don't find out, I will be pissed off...
We also have the fact that this is the guy who was willing to take on (and confident of his ability to, btw) Draconus. :S