Probably they are considering the data for California (relevant to this discussion) rather than the US as a whole. Although the link you posted doesn't break out the percentage for California, it does say that 20% of western state alfalfa is exported.

However, the percentage is less important than the overall amount.

"If California only produced enough almonds for the United States, than it would have about 8 percent more available water. If California only produced enough alfalfa for the U.S., it would have at least 3 percent more available water."

https://www.sandiegouniontribune.com/opinion/commentary/stor...