I think the point is that the design of the SAT influences what is taught in schools.

If the SAT stops testing the ability to deal with multi-paragraph text, then schools will spend less time teaching those skills.

For better or worse, testing was used as an assortative matching between students and universities they applied for. Though far from perfect, a university that set their acceptance bar at a certain level could expect incoming students to at least be at or around that level.

The less rigorous the filtering, the more you have to accommodate the lower ends of the incoming students’ abilities. So as standardized testing is softened, so too is the curriculum that students are exposed to.

There is a movement against standardized testing that gained traction in the past decade, arguing that because it’s flawed and imperfect we should abandon it. The movement never had a good replacement for it, though, so the shift was toward looser standards and judging students based on vibes and non-academic measures. Many of the universities that went this direction are reversing course and adding standardized testing back now because the reality of higher education is that you need to filter incoming students by some academic measurements if you want to be able raise the bar for your curriculum.

The effects cascade everywhere. In a perfect utopia everyone would get individualized perfect tutoring and we wouldn’t have to worry about testing, but in the world we inhabit a lot of the education decisions and realities are downstream of what we can test for.

The implication is not that people learned this skill from the SAT, but rather that not requiring it to score well on the SAT further lowers the baseline.

Doing well on the SAT used to require some measure of reading stamina. It no longer does, so some students who would have been prompted to increase their stamina in order to do well on the SAT will no longer feel that pressure.

That assumes the shift was entirely due to high school students becoming adults. There are also people who haven't read much over those years and have had their skills declined to the point of failure. Or, also likely, sample size issues.

You're making the assumption that the change in absolute terms is entirely driven by deficient additions to the population. It's just as possible that some portion of the population lost their skill by allowing it to atrophy from underuse.