Totally acceptable. Factual. Clear.
Year 10 is an extremely important year. There's so much on the curriculum these days that not everything is done in class. Lots of work is done independently as homework. This is hrem marked and if there's issues, these are reviewed individually or, if it's merited, with the whole class.
If some pupils aren't even attempting the work, then how is a trachersulposed to work out that needs to be covered?
The other glaringly obvious thing is that OP has no idea what may have been previously communicated. Maybe some pupils just aren't getting that for year 10-13 especially, it should be business as usual.
And on the 'what are they doing all day' front - their job. Teachers are having to adapt everything they spent preparing (over last summer break, Christmas break and half terms) so as to be 'teachable' remotely. They are having to review all the work of year 11/13 pupils so they have a fair and reflective GCSE/A level.
There's nothing rude in the message. DS did the work so it's not even aimed at you. Most of the threads on here about teachers are awful - too much, too little, too vague, too detailed bla bla bla. People wanting to moan and take out their frustrations on easy targets.
I hope the teacher sticks to it and doesn't go over it in class. But we know, if needed, they will. Because they're a teacher.