From what I remember of the arguments of Jonathan's erstwhile "prolific contributor", post-RCD additional buoyancy in the Lugger makes it even more difficult to right the boat once it has turtled. Pre-RCD boats were already stable when inverted. With extra buoyancy, the upturned hull floats higher in the water and the amount of mass that needs to be righted is therefore greater.
Apparently, voluntary capsize experiments with Drascombes at a Sail Caledonia many years ago inspired Matt to develop his asymmetric capsize buoyancy system. At least one turtled BR20 owner has successfully deployed this in real world conditions, sailing away unaided from complete inversion in gusty winds in only a few minutes. In fact the whole incident was over so fast that most of us that were there didn't even know that it had happened. I remember wondering why his crew looked so damp.
If extra buoyancy in your Drascombe makes you even more stable when inverted, and in addition you have no leverage because your centreboard has retracted, then as Michael says, you have virtually no hope of righting your boat. Although the Irish authorities don't explicitly say so, the Drascombe skipper might have had a better chance of climbing back on the upturned hull if he and his crew had had something substantial like a centreboard to hold on to.
The report mentions the debate about whether a Drascombe can be successfully righted from inversion without concluding either way. One sensible person on the Drascombe Forum has suggested that the best solution for Drascombes is more buoyancy allied with a masthead float to prevent turtling. That won't solve the problem with swamping and the centreboard slot that Julian describes but it would be an improvement.
What's really sobering is that whatever boat you have been tipped out of, you can quickly succumb to hypothermia in water that is 17°C, especially if you are only wearing lightweight summer clothes. Bala at the moment is just over 10°C and Loch Ness is around 5°C.