I don't think they believe "casual player" = "hermit".
But where they get most of their "ideas" on this are
from their own game data. Store data too now. They use that data to identify trends, to help validate or refute feedback they're getting from a narrow set of people on the forums, to identify which content and features are used, by whom, for how long. I don't have to guess about this, they mention it fairly routinely in their posts and interviews, and one of Turbine's developers told me explicitly back in 2007 or so that they were "big" on backing up opinions with data. I'm sure they're even "bigger" on that now than they were then.
They judged the rework of the epic storyline a success based on how many more people actually participated in it once you could solo the portions which required groups. They were surprised by just how many people did the solo instances they shipped with Moria, and how often... they had to put more limits on them (even once an exploit was plugged) so people wouldn't overstress the servers. They got to see that solo skirmishes were run at least 6X as often as 3-mans, right from the very start. Etc.
Does that mean they should do no group content? No, of course not. But when you have limited resources, you need to be smart about how you spend them. As Sapience put it, if some constituency in your game is 10% of your players, you don't spend 90% of your resources trying to please them. Nor do you spend 0%. (Note that this is over the long haul, not within the confines of one update or expansion... they cannot possibly give everyone something in every content drop).
The problem with raids is that they're
extremely expensive content to build, yet they're consumed by a modest percentage of players. See
this post for a bit of the flavor of just how complex and expensive they are.
Again, that doesn't mean they should stop doing any raids, ever. However, I think it's smart of them to focus on scalable content for Helm's Deep, because it supports most adventuring playstyles, because it will help people find groups more easily, and because it will encourage more players to buy the expansion. "Big Battles" won't be cheap content to produce either, but at least it will have much more reach than a typical raid does.
Khafar