https://www.investopedia.com/terms/c/cashaccounting.asp
This looks at current costs. The school is a cost every year, so every year that cost shows up on the budget. The problem is that road/water/sewer maintenance often doesn't show up on these budgets because these systems are usually built all at once. Because of this they usually also need to be replaced all at once. To see those costs before they happen, you need to use accrual accounting:
https://www.investopedia.com/terms/a/accrualaccounting.asp
The entire message from Strong Towns is exactly that because cities often use cash accounting instead of accrual accounting in their budgetary processes these lingering issues of deferred maintenance don't show up until they do, and when they do, those costs will simply be too large for the city to cover without very politically unpopular interventions.