Ah. I was just speaking from experience. Sometimes when I reinstalled iOS 7 or 8 and it wouldn't give me the credentials, I would have to re-enable autofill in Safari settings.
I noticed that on Yosemite it doesn't seem to 'auto' fill anymore (some sites do, some don't), but if I click in the boxes it will give me the options for that site. Same for iOS 8. However, Keychain still syncs all my credentials.
Maybe they removed the auto-populate feature. Autofill does not necessarily mean automatically fill the boxes on the site when you visit it. On sites where I chose the 'remember me' option it will keep me logged in, like this forum for example. But that is site specific.