The specific rule you point out is stupid but easy to hack. Your score didn't go down because they lost the interest you'd have paid. When you pay off a secured debt, the loaned amount is deducted from your total credit potential, which increases your utilized credit percentage. The hack is to open a line of credit against the secured asset before it's paid off or another line of unsecured credit. Your credit utilization will drop, thus increasing your score.

My score is over 800 and has been for over a decade. I have like 12 credit cards but only use 2 and pay them off every month... Costco for the store and gas and a high cash back card for everything else. The others I keep open with 1 small purchase each year. Every store wants you to have one, so they're easy to get. I have added and paid off multiple small to medium (10-60k) secured loans over the years and my score only fluctuates a little for a few weeks then goes back because my total credit with the dozen credit cards is so large.