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Soy, like most other plants, contain isoflavones

These compounds are toxic, whether they are "poisonous" depends on the concentration.

Yeah, OK now we're playing games. The color in autumn leaves is caused by a flavone. The coloring of a grape is caused by flavones and is an antioxidant and considered good for you. Some flavones have been identified as toxic. Cottonseed, for example, contains gossypol which is an isoflavone. If you eat enough of them you'll get sick. Who'd ever eat cottonseed though? Until your post I'd never heard that soybeans contained flavones.

LD50% (lethal dose) is what we use to determine toxicity. Researchers feed animals doses of something until 50% of the animals die. That's the way in which we express toxicity. Water is LD50%. Virtually anything can be toxic. However, it's almost impossible to get enough of most compounds into an animal to kill it, unless the researchers force feed it.

These are some things that are toxic but you could never consume enough of them to kill you.

Coffee is toxic. It contains about 400 chemicals that are known and another 300 or so that are not. Do you think you could ever consume enough of it for it to kill you?
Rhubarb has oxalic acid. Eat enough of it and it will kill you. No way your body will allow you to do so.
Orange juice contains limonene. It can kill you too. Smells great but it is toxic. Again, your body will never allow you to consume enough to hurt you.

Everything has a toxicity. Whether it reaches LD50% is a different story. To say that soybeans are toxic is technically correct. But, when you consider how much you'd have to consume, with the fact your body wouldn't allow it to happen, makes calling something toxic meaningless.