Without knowing who they define as a "Raider" and how they counted them that is a completely meaningless and useless statistic.
Is it counted over all accounts or only active accounts?
Does it count characters or accounts?
Does the term 'raider' include everybody who has ever tried running a raid, or just those people who run several raids every week?
Does it include people who never run raids as such, but which do ordinary group content on a regular basis?
The answers to those questions, and other like them, make a big difference to what conclusions can be drawn. (And please don't make any assumptions along the line of "Oh, obviously they must mean ....". In my experience that would be unwise.)
Good luck finding answers to them - despite repeated requests on their forums Turbine has never, to the best of my knowledge, gone into any details on how their statistics are derived.
(And for some additional context: The claims from Turbine were given as a reason behind their recent admission to not planning any new traditional group content at all for the foreseeable future. They also claim that the percentage of people they count as 'raiders' have remained fairly constant throughout LOTRO's lifetime, but in previous years that did not prevent them from making raids and group instances.)
