@mathanxiety
I think you're just not understanding what mixed year groups means and how it relates to a long term curriculum. Mixed year group classes are usually with the same teacher for two years (although not always if its a 1.5 year intake or the teacher leaves or moves year groups). They are often in small schools that take on 15 children or fewer per year (although can happen if you take in 45 children, for example).
Imagine you have your child, A, start in their mixed year 1&2 class in 2023 with Mrs Smith, and they start off in September learning about the Stone Age... Great... All good they can carry on teaching in chronological ordered.
But then your child, B, 11 months younger, joins that same year 1&2 class in 2024. What do they teach now? If Mrs Smith teaches the Stone Age again, all of the now year 2s (child A) have already been through that curriculum and have got further along chronologically... and are now back at the Stone Age. So if you move up the time line, all of the 2024 year 1 intake have missed the Stone Age.
Most mixed year group schools have to work on a two-year rolling programme, and that does not work chronologically when you are with the same teacher for two years, but not the same 30 children (15 leave to move up, and 15 new children join)
There are A LOT of small schools across England (if that's the curriculum we're talking about) and many many more across the UK, particularly in very rural areas. I just used mixed year groups across two years as the easiest example, but it gets even harder where a primary school might only have 2 classes and pupils stay in that class for 3 or 4 years with the same teacher (whilst older children move on).
There just isn't a possible way to teach history chronologically in these schools. (Unless you completely separate history out and ask half the class to not listen while the older/ younger half have their history lesson.