I do think there is some covid logic in there too (though agree that childcare needs are likely to be the driving force).
Primaries are used to the one teacher all day set up, so can organise meaningful groups more easily.
To follow the dfe guidelines with y10 and have them in full time, they'd need to be in the same group of 15 all day, with the same teacher. So group A might get really good at German after a month of only being with their German teacher, but will get less input from their English teacher than they would have had at home (because the English teacher might be with group H). I don't get the feeling that the DFE know how to resolve the transmission to teachers issues, so are hoping that schools can work it out themselves.
The suggested solution of teachers going to the classrooms while students stay still isn't effective, because if Angela in group A has coronavirus but is presymptomatic, she could spread it to the whole group and all of their teachers in a week. She then gets a positive test for the virus, and all of her 15 teachers will have to self isolate.
At a primary, in the same scenario, only one teacher needs to self isolate, and that's fine because all of their group are also self isolating so no need for a teacher.
I think, anyway! I may have some flawed thinking here!