Yeah, MU123 is basic (high school) mathematics. The reason for that is that the way the Open University works, they have no formal requirements to join, so they cannot assume that students know even the basic stuff (because they might have left school earlier, or they might be out of formal education for years, or be from a different domain, etc), so the aim of that course is to quickly help you catchup regardless your background.
If you are above this level, you would start with the "intensive start", which skips MU123 (allowing you to pick another module in its place) and then starts with MST124 (precalculus, trigonometry and single-var calculus, roughly), moving you on to MST125 (intro to proofs, number theory, more calculus, linear algebra, etc), in a faster pace.
Yeah I get the part about filling in some missing foundation. But it's the level that surprised me. Some of the MU123 content is covered in 5th grade in the US, i.e. around age 10 or 11.
But I guess there are people who will have forgotten that stuff, if they're not using much math day to day.
If you are above this level, you would start with the "intensive start", which skips MU123 (allowing you to pick another module in its place) and then starts with MST124 (precalculus, trigonometry and single-var calculus, roughly), moving you on to MST125 (intro to proofs, number theory, more calculus, linear algebra, etc), in a faster pace.