Regarding "great" vs. "competent" engineers: every company needs a mix of them. There aren't enough truly great engineers for even just Apple or Google to be entirely staffed by them, much less all of the other companies out there. So everyone needs to make do with merely competent engineers in some roles.
You mentioned in a sibling comment that the engineers in question "contract at Apple"; here you wonder if Apple is merely "hiring competent engineers to implement a roadmap". But that's exactly what all companies hire contractors for, whereas great engineers who are "let loose to think different" are typically permanent employees.
Check out the reviews of Pages 5, which has been dumbed down. If the vision and future for OS X is to turn it into a Fisher Price OS for the masses, I can see why a great engineer would prefer to work elsewhere and tackle bigger challenges.
http://mjtsai.com/blog/2013/10/23/iwork-13-a-huge-regression...
With regards to your other point, no, of course I don't think all Apple engineers are second tier, but I do feel that since Bertrand Serlet left, OS X engineering has languished.
Given that I know some competent - but not great - engineers who work at Apple (and they happen to commute) I wonder if the culture at Apple today is more about hiring competent engineers to implement a roadmap, rather than hiring great engineers and letting them loose to "think different".