That's not what a double-entry accounting system is for. If all you're doing is keeping track of one account/balance, then double-entry doesn't add anything. You might want to still implement it that way for future proofing though if you're implementing an accounting system.

The main thing to takeway is to store transactions like you mentioned (+20, -20). And in the simplest case, just apply all of them based on time.

You're not entirely wrong that double-entry accounting doesn't add much to keeping track of one balance. And the example provided in the article was very simple, just like mine was very simple. Transactions do help, but if you are trying to keep track of a balance and understanding how that balance is changing, double-entry accounting is helpful.