I don't see why the tanker should share blame. The moving exclusion zone extended 1000m in front of it - that's over half a nm. Corby should have been ducking below, not trying to go ahead of the bow. Change in course or not, that boat was not going to stay outside the zone. What would you have the tanker/tugs do..?
I can imagine a series of facts that would indicated shared blame... I am not attesting these as facts, just giving an example.
1. Sailboat is heading perpendicular to tanker's path on a collision course with tanker's starboard side, sail diverts hard to port to cross the tanker's stern on the current course
2. Tanker signals a starboard turn which would be a turn directly into the path of the sailboat
3. The tanker escort crosses sailboat stern suggesting that tanker will cross sailboat stern on their current course so the sailboat diverts hard to starboard to provide more room.
4. Tanker does not turn to starboard in spite of the signal and does not follow his escort.
5. Sailboat freaks out that ship is not turning, diverts 90 degrees to port hoping to run down the starboard side of the ship and cross the stern. Distance is closing fast.
6. Tanker turns hard to starboard to stay in channel, making it impossible for the sailboat to run down the starboard side of the ship and cross the stern
7. At the last minute, sailboat turns hard to starboard to cross the tanker bow, but fails.
Again, I am not suggesting this is what happened, but just pointing out that you cannot tell from the video what actions the sailboat took to avoid collision, nor do you know from the video what information was available to the sailboat about the intended course of the ship.
It is possible that at every stage the sailboat was mindful of the ship and took prudent action to avoid collision based on the best information available about the intended course of the ship. It is also possible that the sailboat never crossed between the ship and the escort because the escort was out of position.
It is really hard to stay out of the way of a turning ship. Simple rules like "constant bearing, decreasing range" do not work. You need to know the ship's intended course, and the ship/escort has a legal obligation to accurately indicate that course. It is not impossible that the tanker failed in its duty.