Decided to read
Sheldon's article again as I didn't remember the details. Looks like the front-only technique is intended mainly for dry, high-grip surfaces.
All text bolded below is my own emphasis
He writes, "The rear brake is O.K. for situations
where traction is poor, or for when your front tire blows, but for stopping on dry pavement, the front brake all by itself provides the maximum stopping power, both in theory and in practice."
He later writes
Originally Posted by sheldon brown
...but there are instances when the rear brake is preferred:
* Slippery surfaces. On good, dry pavement, it is generally impossible to skid the front wheel by braking. On slippery surfaces, however it is possible to do so. It is nearly impossible to recover from a front wheel skid, so if there is a high risk of skidding, you're better off controlling your speed with the rear brake.
* Bumpy surfaces. On rough surfaces, your wheels may actually bounce up into the air. If there is a chance of this, don't use the front brake. If you apply the front brake while the wheel is airborne, it will stop, and coming down on a stopped front wheel is a Very Bad Thing.
So, from what I read, on a downhill on loose surface, you should have been using the rear brake.