The way secondary school accountability works is the average of the reading and maths KS2 SATs scores are taken and nationally after GCSEs are taken the government use that information to show what the average GCSE outcome was for each KS2 starting point.
There is a very lengthy document explaining all this here:
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1112046/Secondary_accountability_measures_2022_guide.pdf
Page 67 shows what I am talking about.
So for example a child with a KS2 average of 110 in 2017 achieved 61.45 Attainment 8 points at GCSE on average in 2022. This forms the basis of the Progress 8 measure and each child's score is compared to their relevant average all this positive and negative figures are brought together and an overall Progress 8 score is produced for the school.
Progress 8 remains the main measure of secondary school accountabilty, for example if you look at the performance tables this is what they they headline with.
So back to my example if the average nationally for a child with 110 is 61.45 then they will most likely be given a GCSE target by the school of a grade 7. This would give them a attainment 8 score of 70 - if a grade 7 was achieved in all subjects (English and maths are doubled in the calculations so essentially it is 10 scores (this is why schools focus on Eng and Maths more)) also of course these are government priorities. So again in this example the child would score roughly 9 points above their attainment 8 target and because it is effectively made of 10 components this is an average of +0.9. It is these scores that are added up so when you see a school that has a progress 8 of +0.5 it means that on average the cohort have achieved half a grade higher per subject than schools with similar cohorts nationally.
I know this is all background blurb but it does explain why schools focus in the way that they do.
So.... schools are ingrained in this way of working however, it doesn't work for every child, at best it is a loose guide.
Why does this exist? Well before progress measures the main headline measures on which schools were judged was things like % 5+A*-C grades including English and maths. This unfortunately created a massive focus on the C/D grade borderline and the focus that undoubtedly happened would be that a lot of the effort and resource would go into these groups to get C grades. This potentially was at the detriment of those who were comfortably a B grade for example or an A, they weren't pushed as much to achieve higher.
Under progress 8, whilst it is mostly data nonsense it does mean that there is incentive to try to improve every grade for every student.
HOWEVER, as you rightly say, your child has no official SATS due to the pandemic. So all of the above doesn't apply. The government haven't yet said what they will do about this, but what can they do? Progress accountability measures only work when there is a start and end point. Therefore it is likely that for these cohorts we will be back to the attainment only measures and potentially the grade boundary cliff faces they present. Nonetheless the school will be sticking to what they know in terms of flight paths and targets (I'm not saying any of these things are right but they do provide a framework for the school to see who needs more support). The next problem is that they have no SATS so the school has made them up. They should have an assessment policy on exactly how they have made them up but common ways are either to get a prediction from the primary school on what they would/could have achieved at KS2 or they will have done their own round of tests in Year 7, either SATS or CATS and tried to convert them to give each child a starting point. Across a whole cohort this is possibly OK but at individual child level it is very problematic.
Therefore whilst all this exists because it is the crutch by which schools work, the teachers SHOULD know some of the above and that it is all nonsense really at individual level and they will be pushing each child to be the best they can be. Teachers of course are professionals and they should be trusted to do this. Only if you feel the school policy is interfering with teacher professional capabilities then this is an issue.
Again you could challenge the arbitrary starting point for your child with whoever is in charge of assessment at the school because there is no basis in fact for it, the school can't be judged on it (for your year group) so really they have the power to adjust it if they feel it is relevant.
Sorry for massive post and good luck.