The pictures on this site indicate the lines are terminated by zig-zag sewing rather than fig 8 knots?
Is this a recognized way of terminating q-power line?
no it's not. the specs on q-powerline (or any other cored line) require a knotted end or sleeved and knotted. The reason is that if you sew the ends all the load on the line is going to go on your stitching.
the stitching doesn't carry ALL the load, it only carry a fraction and only when the line loosens. Larks head/reef knots when tight strangle themselves preventing the line from slipping the stitches only need to hold this slipping force. which is why you can get away with not using spectra sewing thread, and imperfect sewing.
Try the following. Take unstitched q power, tie a larks head. tighten it good onto something. leave the end you'd normally tie or stitch loose. you can now hang of the line if it's tight enough despite having NO stitching or knots at all on the free end. loosen it up a little and it'll slip
This should give you a bit more confidence if you F up a stitch or 2.
If you're still concerned the Canadian distributor of q power can also get you some spectra thread. I've hack tested a few lines even with intentionally sketchy stitching with regular polyester sewing thread and my termination point is not normally the source of failure.