Because that sentence doesn't apply to either Bogaerts or Betts. Both spent at least some time in AAA (even if it was brief) and demonstrated some success there before being called up. Bradley in early 2013 skipped straight to the majors from AA, which is what some people seem to want to do with Benintendi...that, to me, makes him a much more relevant example.
Of course, as you say, we can't say for a fact that Bradley's rushed initial call-up contributed to his early struggles in MLB, and there others like Conforto who have bypassed AAA and gone on to thrive in the majors. So it's all relative. I'm not against promoting Benintendi aggressively (if anything, I wished they'd moved him through the lower levels a little quicker), I just see no need for him to go straight from Portland to Boston at this juncture. Ultimately, though, I trust the team to make the right call here.