While I agree that it's annoying that WEG and Daedelic get overlooked, I think the gist of the article is probably right. It seems like adventure games were something of a mass phenomenon in the past, and (to the extent you consider TellTale games to be a mass phenomenon) are returning to that status now. WEG and Daedelic make niche products, by comparison.
That said, when I was trying to find data to back this up, I was surprised at how relatively low
Lucas Arts sales figures seem to be -- perhaps they're inaccurate, though. [EDIT:
This site also reports low figures.] Apparently even KQV only sold 500,000 copies, although granted that was probably for $40 a pop or whatever.