Here is the actual quote:
"Oxalic acid is a natural product found in spinach and some other plant foods including rhubarb. (Levels are so high in rhubarb leaves that we don't eat them - they're poisonous). It imparts a sharp taste to beet greens and chard that I don't like, especially in older leaves. Concentrations of oxalic acid are pretty low in most plants and plant-based foods, but there's enough in spinach, chard and beet greens to interfere with the absorption of the calcium these plants also contain. For example, although the calcium content of spinach is 115 mg per half cup cooked, because of the interference of oxalic acid, you would have to eat more than 16 cups of raw or more than eight cups of cooked spinach to get the amount of calcium available in one cup of yogurt."
Andrew Weil, MD
http://www.drweil.com/drw/u/QAA400344/Avoid-Vegetables-with-Oxalic-Acid.html
Note that he states the interference with the absorption of calcium is of the calcium contained in the plant itself. He goes on to state that it typically will not interfere with the absorption of calcium consumed at the same time from other sources. Note also that he states you would have to eat "more than 16 cups of raw or more than eight cups of cooked spinach" in order to absorb the amount of calcium in one cup of yogurt.
So, it would appear, based on Dr. Weil's statement, that spinach would be o.k. as a treat so long as you were not expecting it to provide a source of vegetable calcium to your birds. I, however, will continue to err on the side of caution and not give it to my flock as a treat or otherwise. There are plenty of other treats that they enjoy as much or more without risking anything in their health.