I think this has been the prevailing wisdom for some time now, whether or not it is still true. I also think this was more true in the 3.x days; Morrus recently posted some data that tended to debunk the prevailing notion that visitors to enworld are all hard core greybeard grognards and that younger players actually visit the site quite frequently (though I don't think the data was limited to posters, just site visitors).
This seems to have been the case, though I don't have the data and quotes to back it up. They certainly did not seem to invest in the market and customer research that they did w/ the 5e play test (I say that as someone who has nothing, or very little, against 4e and enjoyed it). But even slightly before that time, the luminaries of the RPG design had drawn way back with their online posting presence; especially compared to the days when Monte Cook and such would post fairly regularly and openly on the forums. During the 4e days, only the Brand Manager (Scott Russ?) and his assistant would post openly and semi-regularly on the forums and even that dwindled as time went by. I think it is all part of the online digital cultural evolution that is happening around us every day. Companies and brands have learned that it is better to use broad, official social media accounts to address the masses directly rather than risk 'rolling in the mud' (as can happen frequently) in online forums.
I still think online forums would be useful in 'stress testing' your system, as it were, if such feedback could be taken and applied properly, so I agree there about 'tweeks'. But I also suspect that the so called disparity between forums (at least certain forums) and 'regular play' is not as great as it once was as well.