I am not one of those experts but here is my two cents:
The 12, 15 (or however many months) they are referring to is the time between your medical and your FILING of I485. As long as you file I485 and take your medical approximately the same time (in theory within the X months, but you don't want to wait that long to file I485), you will be fine. I know people who got their I485 approval after 2-3 years and they didn't have to re-take their medical.