If all routers in an AS are BGP speakers, then there is no need to have any interaction between BGP and an IGP. In such cases, all routers in the AS already have full information of all BGP routes. The IGP is then only used for routing within the AS, and no BGP routes are imported into the IGP.
For routers to operate in this fashion, they must be able to perform a recursive lookup in their routing table. The first lookup will use a BGP route to establish the exit router, while the second lookup will determine the IGP path to the exit router.
Since the IGP carries no external information in this scenario, all routers in the AS will have converged as soon as all BGP speakers have new information about this route. Since there is no need to delay for the IGP to converge, an implementation may advertise these routes without further delay due to the IGP.