Yes I think they are conflating square pixels with square pixel aspect ratios.

If a video file only stores a singular color value for each pixel, why does it care what shape the pixel is in when it's displayed? It would be filled in with the single color value regardless.

Because if that pixel takes up 2 vertical pixels when displayed in your web browser... That takes up more space and causes layout shift.

I thought i understood the article just fine but these comments are confusing.