When I run init() in devtools console while using the docs, then date-fns functions become available under _ variable.

Nice, but... it looks like the functions don't have all the features. Maybe it's outdated version? The latest (3.6.0) is selected in the drop-down on top of docs.

Notice how the startOfDay function doesn't understand the ISO string argument:

I can run this just fine in my own code:
startOfDay(new Date().toISOString()) -> Sat Aug 17 2024 00:00:00 GMT+0200 (Central European Summer Time)