The page is responsive. The sidenotes seem to always be available at the bottom as footnotes, but if your browser window is wide enough they also show up beside the location they appear in the text.
I'm not seeing them as sidenotes with a maximised browser window on either my landscape (2560x1440, no scaling, which I'd consider wide) or portrait (1080x1920 – though that being thin I'd not expect width sensitive things to be present there) screens. Just foot-notes and pop-overs when interacted with.
I see them as sidenotes on the desktop.
The notes showing up along the side is dependent on page width