I'm sure you may be familiar with this but wouldn't you calculate your cash on cash return by taking your worst case cash flow for the year $18,000 ($1,500 x 12 months) and divide by $150,000 (total investment) which is a 12% return.
So essentially if your interest rate on your HELOC has under a 10% interest rate you're using positive leverage to create a higher expectation by borrowing that cash. Goes without saying the higher the cash on cash return and lower the interest rate, the better.
DR's advice is targeted at an average consumer, you can grow wealth faster by using debt in your favor.