I believe that both COVID and LLMs are disrupting educational metrics. But in terms of which one did more short-term scale damage (i.e, within 4 years), I would wager it was COVID.
A continuous decline across both COVID and llms points towards a different causal relationship to me. I would be more likely to believe algorithmic social media or computerized teaching tools are to blame, but that is pure conjecture.
I think the pandemic had some effect on the current cohort of 15 year olds, but it should not be as large of an effect as the students who were 15 in 2020.
If I missed a week of math class at age ten would it have the same effect on a test given when I was 15 as if I had missed a week of math class just before the test? Your position seems to be that missing math class at any point in your schooling is a permanent unrecoverable setback
A student who is bad at arithmetic will tend to struggle to learn trigonometry. A good teacher can realize this and allocate time to arithmetic practice instead. But this time has to come somewhere. As long as the number of teaching hours remains the same, they have to sacrifice something else. So if the students are all caught up in arithmetic, they will be behind in trigonometry by the same amount.
To fully catch up without sacrifices, students who studied less would need to study correspondingly more to compensate, but there was no concerted effort to make that happen, although the opportunity certainly existed.