Well, I didn't say they didn't see the tanker, but like you wrote, probably didn't think it was moving as fast as it was (they move faster than you think) and/or simply weren't paying attention to their positions. Whatever the reason the captain in the sailboat is at fault since large boats in the shipping channel have the right of way, or might makes right in this case, and add a whole new dimension to racing.
The 16 in the photo was not far behind me in a race on San Francisco Bay when we needed to gybe in high winds and cross the shipping channel to make the downwind mark when the tanker was about three miles away. I suggested waiting five minutes for the boat to pass but my crew had confidence we would make the gybe, and we did, but the 16 didn't. The scuttlebutt ashore was the tanker was able to change course in time while the safety boats towed the 16 as best they could.