The guy is full of crap......Douglas fir is worthless for backing, and not much use in belly wood either. Doug fir heartwood "can" make a bow, but there are way to many choices here on the west coast to mess with fir. Old growth will make good arrows, not bows. Exceptions would be under glass. 
Still don't see a point.
If you have yew, and oak .....yew would be the belly, oak the back in my opinion. 
VMB
I don't know who this guy is or anything and I am not gonna say anything bad about him or his experience, but the fir I have used (in 2 x 4 lumber form and all) definitely doesn't strike me as a tension strong wood? It seems brittle if anything. But of course, I have never used it as a backing, so I honestly don't know. I have been using some hard maple lately though I picked up for super cheap, it has worked just great. I am loving it, nice tight grain, nice and bendy and tension strong, and also light in weight compared to hickory or bamboo. That has got to be my favorite part. I haven't been able to break a bow made with it yet.
Ok so I goofed up; the Douglas fir is used under [Glass]. So forget I even mentioned it..
Like bubby said, sticking with oak and maple is best bet, easy to get and cheap. I did a test glue up with the Yew yesterday, went out to the garage and pulled the limb she snapped easier than bending a toothpick. So the yew is out of the question, to dry and very brittle, only good for handles and tip overlays, I'll pick-up some maple.
"Roy that bow looks really nice. "
