That ships are at anchor in an area at any one time doesn’t make it a “recognized anchorage”.
The reason why several vessels were at anchor there at this time COULD be congestion at the designated anchorage and / or at the Humber Ports.
The fact remains that it is an open anchorage, not a regulated one, as it is outside of the Humber VTS area.
None of that oblive any ships from keeping a good lookout and situation awareness and take such actions as necessary to avoid a collision by any means available.
Obviously in this case one vessel had a lot more means of avoidance than the other.