I've had to debunk this myth multiple times in this post already, I'm not sure who started it. Legumes do have all the aminos and in sufficient amounts.
Also on bioavailability, it is a double edged sword. For example heme uptake is greater than mineral iron, but your body has very little control with inhibiting uptake when you already have adequate levels. With the mineral form your body has various ways to promote or inhibit uptake. The same is true of your example of vitamin A. You can pretty much eat as many carrots as you want and suffer no ill effects, eating too much liver or taking too many liver oil supplements however can lead to poisoning.