They are not hijacking anything, imo. They are simply using Notion.so to power the website. It's a note-taking app and this is the regular behavior of Notion. Nothing out of the ordinary.
Down key makes you navigate "blocks". Space key doesn't do anything since you're not typing in Notion.
There's a way for the OP to get around this behavior and make their website act more like a blog.
That would require SSR on Notion pages instead of a cloudflare worker script on their public Notion site which they appear to be doing currently.
Well, Notion is hijacking it then, but the difference as a reader is irrelevant, the point is that I cannot press neither space nor down to scroll down (if you scroll manually then press down, it'll jump to the "highlighted" paragraph, no scroll down).
Down key makes you navigate "blocks". Space key doesn't do anything since you're not typing in Notion.
There's a way for the OP to get around this behavior and make their website act more like a blog.
That would require SSR on Notion pages instead of a cloudflare worker script on their public Notion site which they appear to be doing currently.