I've made this recipe three times now and have never used a thickener. In fact, I substitute honey for white sugar, cup for cup, and I still don't need a thickener. You'll want to heat the jam on medium-low for the first half hour, then medium for the next hour and a half. You want it to be simmering pretty good or else the liquid cannot reduce. Two full hours on the stove with a good simmer produces a lovely jam that thickens more as it cools, and after a couple of hours in the fridge, it's perfect for spreading. The first batch I made, I added thinly sliced fresh lemon basil in the last 5 minutes of cooking. The second time, I added a couple of peaches and an extra 1/4 cup of honey. Today, my third time making the recipe, I added a few pinches of cinnamon. It's excellent jam. Don't be scared off by the 1-star review. You DON'T need a thickener. Just be sure it's bubbling away on the stove for that last hour and a half and you'll have a great jam!
unfortunately it does not thicken enough to be a spread. this is only good for a topping. you must use a thickener. I tried corn starch and did not like the flavor. I wanted a recipe without pectin.