3.2g of arginine in a day doesn't sound like a lot. There's around 2g in most meat/protein servings, and 1g on each egg yolk[0]. I wonder how much of the study outcome is just the result of changing dietary habits of the patients.
It's well known that elderly people have reduced ability to absorb protein through the gut. I'm just speculating here but could that be one of several reasons why elderly patients are at higher risk of COVID-19? Perhaps some of them are simply deficient in essential amino acids.
[0] http://www.dietandfitnesstoday.com/arginine.php