To me that seems a bit longer than the context of what was reported by Hendrickson. It seems that Hendrickson came topside right after the impact, along with most of those in the forecastle, and went back below about 5 minutes later thinking not much really happened. He saw the ship was still making way when he looked over the side as well as the berg disappearing aft into the dark off the starboard quarter before going back below. He then said he went to turn in again but one his mates (Ford) came by to inform him that the bottom of the tunnel was flooding. He then went to have a look and saw water coming in from the starboard side at the bottom of the stairs. He was on G deck at the time. I'd put the time as about 10 minutes post impact when he observed this.Discovery came about 15 minutes later when water had not yet filled the tunnel.
I also think that Garzke's explanation that was posted by Ioannis above is most plausible.