The article mentions the content is 500mg of oxalic acid / 100g and says the deadly dose is at 15 - 30g. That makes for 3-6kg of rhubarb leaves. Quite a serving, if you ask me.
In the original British understatement:
> Since a lethal dose of oxalic acid is somewhere between 15 and 30 grams, you’d have to eat several pounds of rhubarb leaves at a sitting to reach a toxic oxalic acid level, which is a lot more rhubarb leaves than most people care to consume.