The writer has chosen to use the expression won't scale to mean that today's methods of food production are not scalable.
Something is scalable if increasing productivity follows a straightforward ratio or scale. For example, if it takes one man to dig one hole in a week, then you will need five men to dig five holes in the same time. The assuption is that each man could dig one hole, so that the five holes can be dug in parallel. This same situation would not be scalable if they only had one spade, so only one man can be digging at any time.
In the case of food production, we have progressively more people who will need (and could produce) food but the planet isn't going to get any bigger so we won't have sufficent land, water etc.