Although the article talks about the issue of first grade readiness, I think the bigger problem schools are going to face is that there will be an enormous range of returning students.
Some students will have had very little academic exposure (due to lack of technology/broadband/area for learning) and some students will have had their education supplemented by their parents. The impact of these supplements can be tremendous, since they are personalized to the specific student. Also, students have more spare time out of school, since Zoom school takes up fewer hours than regular school (no bus ride, shorter lunch, no time spent lining up to wash hands or getting backpacks).
Schools will have to decide how they want to deal with the great disparities that they will see next fall (across all grade levels). My guess is there will be much less stigma attached to repeating a grade than in normal times. I hope this is the case, because it would not be in the best interest of under-performing students to pass them along to the next grade if they're not ready.