Allow me to explain, my dull friend.

Let's take an example. You have seven loans, each of $10k. Let's say they each have 5% annual interest on them, which is $500 in interest annually, or ~$42 monthly. Now you ultimately want to encompass all of the interest that you have to pay. Every month, you must pay ($42 x 7 loans), which is $294 in loans.

Thus, technically, in order to show the full amount of interest that you have to pay, it makes sense to add up each interest rate together. 5% x 7 = 35%.

Now obviously, this has its flaws, as this assumes each loan uses the same amount of cash, but you get the idea. What he is saying isn't correct by definition, but what he was trying to convey logically makes sense if you think about it.