Ironically, in the list of examples of how to use <i> under Usage Notes, they actually use <em> instead.
In this specific case, it’s probably just a bad mechanical translation, possibly from long ago as part of a batch i → em change, or possibly recent in their conversion from HTML to Markdown (might have done both em and i → *). Ill-conceived in either case, but not the end of the world.
If MDN docs is autogenerated, that's regrettable. I swear I'm not going to fall back on W3Schools.
Incidentally, whatever happened to W3Schools? There was a time when every google search for anything remotely similar to web development came up with the top 5 links being to W3Schools. Nowadays not so much. Did google finally decide that W3Scools content amounted to disinformation?