I like this. False suggests it is objectively false, but you're right, it really depends on more information. And it's important that people working with dates and times understand that they need more information.
The point however is that every dev sometimes needs to make assumptions about an calendar — even those who are smart enough not to write their own calendar. E.g. if you think about a typical fitness tracker of course the devs will have to make assumptions about weeks and months, instead of supprting every calendar under the sun.
This is why a Depends makes a lot more sense here: Knowing that it exists is good, but you might not have to support it in an 1.0 release
It's a blog post, how can a "false" make you feel scolded?
If it is false only if you pinch hard and look from the right angle it comes of as "Oh come on, you just don't want me to get this right" — which in this case runs counter to the whole idea of the blog post as I understood it.
I had a teacher once who was like that: whenever someone he didn't like got something wrong he would pull out some pedantic detail to still make it look like the person was false, even though they really really were not. I kinda think this is just a little bit unproductive in an educational context.
So I think instead of "scolding" the reader with a False here, maybe a Depends would be both more friendly and more well suited.