You're not doing anything wrong, I just don't think you're using the term backsweetening correctly.
You're using 1 can of concentrate for five gallons, correct? If so, it is fermenting, you're just not really noticing it.
Backsweetening is done to raise FG to whatever level; and it can only be done by making sure the yeast don't eat the new sugar (using the methods I mentioned before), or by using non-fermentable sweetners.
The OP in your non-HBT link says when asked
Originally Posted by question
Do you add campden tabs to keep it from refermenting?
Originally Posted by answer
yes i campden and sorbate it
And, the HBT linked recipe doesn't include any backsweetening.
I just want you and others to know that adding more fermentables after the fermentation is "done" (without doing anything else) is going to restart fermentation. So, if you end up bottling immediately, you could wind up with bottle bombs (note: like I said before, it shouldn't be an issue with one can of concentrate per 5 gallons).
Don't get me wrong; I'm not trying to be critical, and I'm gonna try this recipe