During the commentary track for Doctor Strange, as reported by Comic Book Resources, Scott Derrickson confirmed that, during the psychedelic sequence that occurred when The Ancient One opened up Stephen Strange’s mind, the Quantum Realm does appear, which was first seen in Ant-Man.
It is interesting that, even prior to the release of Doctor Strange, Marvel was able to tease this multi-verse idea in Ant-Man.
Even before director Scott Derrickson confirmed this connection, Ant-Man and Doctor Strange did share some connections.
Really attune Doctor Strange fans may have even been expecting there to be a bit of an Easter egg for Doctor Strange during the Quantum realm sequence in Ant-Man, and now in hindsight, it sort of serves that purpose.