That's partly true, but it also has to do with structural changes that have changed the wind patterns, or so I've heard from those way smarter than me.
It's near impossible to hit one out in Fenway (Drew's PECOTA projects 12 HR this year!), but you can rack up doubles big time. It's plays neutral I guess, if you consider neutral to be one foot in ice water and one on fire.
Looking at ESPN's park factors, I can see where you are coming from. I would caution however that park factors are extremely susceptible to yearly surges. A lot of the park factor is David Ortiz hitting ten more home runs on the road and Josh Beckett's gopherballing. Looking at three year park factors (2003-05), Fenway Park is a very tough home run park for lefties, but a great home run park for righties.
I don't know if it's true or not, but my uncle told me once that the Yankees and Red Sox once worked on a Ted Williams for Joe DiMaggio deal, for the lefty advantage in Yankee Stadium and the righty advantage in Fenway.