So 628 liters of water for the first liter of milk then another 628 liters for the second liter? Seems like your math is a little wrong.
Just googling around, growing one pound of oats takes about 290 gallons. It takes about 1.25 cups of oats to make a liter of homemade oatmilk. 1.25 cups of oats is about .25 pounds. That's 72.5 gallons to get your one liter of oatmilk. Or about 274 liters of water.
I have no problem with people going to better alternatives for health and environmentalism and more - even tastiness, but that chart seems a little wrong.
https://www.huffpost.com/entry/food-water-footprint_n_5952862 https://minimalistbaker.com/make-oat-milk/ https://www.traditionaloven.com/conversions_of_measures/rolled_oats_measures_converter.html
But at what cost!?