No, it has nothing to do with a fast metabolism. Your food is still in there digesting, and potentially interfering with your ZMA, after 15 minutes.
A couple points:
1) Insulin blunts the release of GH. Food causes insulin release. Not great immediately before bed.
2) Calcium interferes with ZMA, so whey/casein/milk/cottage cheese within 2 hours of ZMA is not great for optimal ZMA effects.
3) IF you're crunched for time, say getting home from a workout at 10pm, and needing to be in bed by 11: take the ZMK at 10pm, wait 45 minutes, then eat some eggs, tuna, chicken, or some other form of protein without calcium. And if you want maximal GH release, avoid carbs during that time.