You're both right, to some extent. Many (if not most) IMG's match without full ECFMG certification. However, some programs may want the full certification prior to ranking / interview. Why? If a PD takes an IMG who then runs into delays in getting ECFMG, then that delays visas, etc, can run into a really late start. Once that happens once, PD's may tend to avoid it in the future. Same with someone who says they will be a citizen by marriage -- I've seen that take 6-9 months to process, and I'm never making that mistake again.
So, bottom line, most residencies won't care and will take you with just ECFMG verification. More competitive fields / programs, or perhaps programs that have been burned in the past, may prefer full ECFMG certification. It's a double edged sword though - if you're certified, it means you have a longer break in your training which may be seen as a negative. The "sweet spot" would be graduating in December -- then you'd be certified, be able to interview freely in Jan and Feb, start in July, and not have a long break.