LH hitters feast on Fenway: David Ortiz, Mike Greenwell, Trot Nixon, Johnny Damon, Scott Cooper (Fenway made his entire career), Fred Lynn, Otis Nixon (switch-hitter, but his numbers as LH hitter during his season in Boston were far better), Troy O'Leary, Billy Goodman, Johnny Pesky, Doc Cramer. There's a century's worth of LH bats - slap hitters, power hitters and everything in between - who have enjoyed the BA benefits of Fenway Park. It's a great hitters park for everyone - no foul territory, big RF, deep to dead center, and you can bang hits off the wall no matter which side you hit from.
And Pagan is no more of a slap hitter than Ellsbury is. They have an identical career OPS+ of 106. Neither takes a lot of walks (Pagan has a career 7.4% BB rate, Ellsbury is a 6.7%). Pagan even has a slightly higher career XBH% - 8.3% compared to 7.9%.
I was actually being kind to Ellsbury when I said that outside of 2011 he's Angel Pagan. The truth is, if you wiped out Ellsbury's 2011 season, Pagan's got a clear advantage. Pagan doesn't have a lot of power, but he's been more consistent with it.
Clearly you have mentally sorted these two in different boxes. You shouldn't have. Both are BA-driven players with nearly identical career percentages (at least when you adjust park-adjust them). Baseball-Reference even lists Pagan as the second-most comparable player to Ellsbury.