The workbook method works for kids who are self motivated and can learn from reading and practicing. Which isn't a lot of kids. Those self learners are probably way ahead of that workbook anyway.
We send our kids to school to learn with others (at least I do), to share the learning experience and bounce ideas off each other, led by a teacher. Someone who can take those ideas and shape the kids into better thinkers.
Your issue is my pet peeve. Yeah, it's one way to work "at your own level". And it's good they are teaching the units too, but how much actual teaching time is there? And if there isn't much, how much confidence does this teacher have? (Our GT teachers have admitted they are afraid of not being able to handle the GT students so their response is always "go home and look it up and then tell us"--which is maybe better than the workbook method, but still is not teaching the kids!)
Yeah, dd isn't really self-motivated. She will do her work but to the bare minimum that is required. The workbooks don't even have instruction in them, if they need to understand a concept they have to go get a reference book off the shelf and look up what they need help with but very few of the kids actually do that.
ETA: We had a day where we go in and dd gives us a tour of her classroom and shows us what she is doing. She showed me her math workbook and the pages she had done ahead of where her math group is and some of the problems were wrong. I asked dd how she got the answer and she said she guessed. I asked her teacher about this and she said that that is ok, that they can guess. The problem is I don't see them getting to that unit and correcting dd's guesses any time soon.
It's ok for kids to make mistakes but if you don't correct them and teach them the right way how will they learn it? They're just going to learn it wrong. I'm not a teacher but this method just seems like a bad idea.
