I’ll be the first one to state that clipless or clip pedals are a 100% factual mechanical advantage.
But that does not mean that they are a racing advantage. I found myself holding back too much when clipped in. Worried about going OTB or looping. In flats I could give a rats ass about that as I know I can handle it not clipped in. So technically I am faster in flats. The clipless setup was a disadvantage to me. But I also grew up in the era that was preclipless. So I will be riding flats for now.
For the new riders who have started in the clipless era they have a different problem. They feel uncomfortable on flats and may also be scared of clipless. They may never reach their full potential as they do not feel 100% comfortable with letting it all hang out on a set of flats and seeing what they are really capable of doing.
It is like saying a short rear triangle on a bike is an advantage. It is a mechanical advantage (easier to pull the front end) but is it a racing advantage? Too short and you will loop out, not feel confident taking a jump at speed etc.
Get my point?