I believe the author misses an important step. Once the oil has been removed from the algae, the now dead algae can be further processed to recover nutrients such as phosphate. The recycled phosphate can then be added back into the cycle and reused by the next batch of algae.
The author also makes the incorrect assumption that the land area must compete with current farm production land. This is also not true. Algae can grow in areas not currently production lands. Semi-arid or “dry” land can be used for algae production and that land is only marginally viable for farming. Given the right infrastructure, it is even possible to grow algae in desert or arid lands.
1 000 000 hectares is about 10,000 square miles or about 100x100 miles. That’s with a single layer of water, 1 ft deep.
If you were to put two layers, each one foot deep, one top of each other, you would reduce your square area to 5000 square miles, or 70x70 miles. Go for three layers and you have an area of about 55x55 miles.
I don’t know about the phosphorus part, but the areaa that is needed to use algae is not insurmountable.