As far as I know, valve do not ban for using achievement modding tools in single player games.
I used one for fallout 3 a while back after the game decided to not give me any achievements. I accidentally unlocked them all at the same time. It's been about 6 years so I would expect that they would have banned me if it violated the terms.
Still, don't do it. It's just not worth it. If you have any suspicion that it may ban you it's probably best not to do it.