It's actually not false. If you read the article you posted, you will see that half of the water is used for environmental uses. That means is not used by humans, but "let go".
So if you sum the rest, agriculture does use 80% of the water used by humans.
Directing water to a tree to grow almonds, and directing it to a stream to support the delta smelt are both human directed usages. You can't just count the usages you don't like to make the number look big.
If you're looking at it this way, that every single stream is managed, then yes, you're right. I rather think that we develop only the water sources that make the most sense and leave the rest be.
This way we could talk about the incredible amounts of gold that is diluted in oceans that we could harvest "anytime" [0]. It's around 21x more than humans mined in the whole history.
"Solely relying on these statewide volumes is controversial because they don't consider the fact that most of the volume of water used for environmental purposes includes flows down Wild and Scenic Rivers in the North Coast where there is no practical way to recover it for either agricultural or urban use because it lacks many connections to the statewide water supply system."
In an average year, about 39% of California's water consumption, or 34.1 million acre-feet, is used for agricultural purposes.
https://en.wikipedia.org/wiki/Water_in_California#Uses_of_wa...