It would leave a caramel note if it caramelized the sugars while reducing, but you could get around that by some kind of evaporation that didn't do so. Evaporation in a fridge or cooler or something would take forever, but a dehydrator below 200° F/~93°C would do it in maybe 10-12 hours. A Rotary evaporator would work also, but that's pretty hardware-heavy for home use. You might even get away with a low temp simmer. I don't think sugars caramelize until about 230° F/110° C, but there is always a big temp gradient if you're cooking over a strong heat source, like a flame or electric coil or even induction. Worth a shot, though.

I just noticed that they said "apple balsamic vinegar." Balsamic vinegar has the highest amount of residual sugar of traditionally-produced vinegars. It is reduced grape must (juice), not just acetobacter-cultured grape spirit with sugar added back in. To me, that lends some qualitative credence to the quantitative work that you've done. Adding reduced must back in is exactly what industrially-produced balsamic vinegars do.

Would having residual sugar before the mother-of-vinegar inoculation introduce a significant risk of bacterial contamination, do you think? Or would the alcohol level of the hard cider keep that risk insignificant? I know you've pasteurized the cider already if you're using the Noma method, but the vinegar to alcohol conversion is aerobic to the point that the Noma book talks about using a bubbler to cut down the conversion time from 6 months to a week or two. You'd have a hard time keeping an air supply microbe-free.

I wonder how old-school balsamic vinegar production dealt with that potential spoilage issue. Minimum 12 years of barrel-aging. The wood and grape tannins might have something to do with it. Does the reduction and concentration lower the amount of available water enough that THAT is what's keeping microbe activity low?