Market timing = rejigging your portfolio based on what you think the market will do next.
Rebalancing = restoring your original/planned asset allocation when your portfolio has strayed too far from it over the course of time.
If you've decided an 85/15 split suits your risk tolerance, and over time that becomes a 90/10 split, rebalancing back to 85/15 isn't changing course to time the market, it's getting back into your comfort zone, which you presumably carefully picked for good reason. The idea is you're not going 'well since the stocks are growing so much faster, why not let them keep at it and keep giving me these delicious delicious high returns' until you're at 99% right before a crash and THEN you panic and sell some to buy more of your suddenly more attractive bonds and oh look you have done this All Wrong.
Deciding to go for 40/60 because you think the market is about to crash is definitely market timing and history shows you're more likely to be wrong than right. Even if you were right and experience a crash soonish, you're likely to miss out on some gainz along the way up to it, and you're unlikely to time your re-entry into shares accurately either.
You're correct that rebalancing does provide a mechanism for, broadly speaking, buying low and selling high. It's inherent in the system, whereby the parts of your portfolio that are growing fast at any given time get pruned and fed to the classes that aren't doing as well. It doesn't do so as accurately as if you were a hindsight-level perfect market timer, but it also doesn't do so as badly as if you TRIED to be a perfect market timer and to guess when something is a high or a low while in the thick of it.
It might seem like the difference is meaningless since it's all about your plans and intentions (you can't look at a trade out of context and see if it's a foolhardy attempt to time or a planned rebalancing) but it's actually essential because it dictates your behaviour, and you're the one in the best position to fuck things up for yourself.