Not dumb... It’s more dumb that the directions state that when they know it’s not a common occurrence for a regular DIYer to be in situations where they are tq’ing to spec an open end wrench, lol.
I’m sure most end users just got it “that’s good” tight.
If you want to get a TQ wrench on there, they have adapter type tools that hook on to a wrench and then have your hole for the. 3/8 or 1/4 drive, but I would question how that affects the rating due to altered leverage.
do you have any crows foot sockets? You can use that on your wrench, but also...it’s a jam nut on that strut bar, just tighten it up and it will be fine. Keep in mind 15ftlb is not much, so you don’t need to stress about trying to get it super tight.