My point was that AIS is not an anticollision mechanism and shouldn't be relied upon. It's also often wrongly configured or just broken.

The Royal Navy advertise their 8500ton Destroyer as having the radar cross section of a small fishing vessel: https://navynews.co.uk/archive/news/item/3951

I'm not sure what the weather conditions were like, but even in marginal conditions, they'll have been picked up on radar.