As far as I know, nobody throttles CPU while on battery, but you never know. I don't recall seeing reviewers post separate test scores for battery vs. AC power; you'd think they'd do that if there was anything meaningful to be learned. For that matter, if throttling on battery was a known practice, consumer protection law would likely require that to be noted in the manufacturer's documentation - if a portable computer only achieves the advertised specifications when connected to AC power, that could be considered a deceptive sales practice.
If you have a 2017 MBA, why not download and run Geekbench, and see if you get different scores when connected to power?