Pixels are not squares. Pixels are sample points, arranged on a square grid. One of the things you can do with those sample points is fill in the whole square around them in the color of that sample point. But that is not 'what pixels are'.
In the context of what they said, pixels are the smallest unit of a display, all you will ever see with your eyes if looking at a display, and they are almost always square.
Having them represented as a circle or diamond wont change the point they were making.
They’re not actually square, though. They’re (on almost any display technology) a mosaic of red green and blue patches arranged inside a square.
On a ‘retina’ class display these square areas are explicitly smaller than the resolution distinguishable by your eyes.
Any sharp edges or corners you think you can discern are your mind reconstructing an edge from a series of neural samples on your retina that are the sum of the output of various sub pixel blobs of red green and blue light that focus on that particular cluster of cone cells.
