Not really. You can't, for example, lift weights 8 or 12 hours a day unless you're taking steroids because your muscles need to heal and they can only do that between bouts. You can through-hike long backpacking trails but it takes a toll on your body and eventually you will have to rest. And you can't hike all day and into the night, you need to rest between bouts, again because your body needs to heal.

I think the dangerous thing about people thinking you have to code all the time is that they're just assuming that it makes you a better coder without any real reasoning. And that's the kind of assumption that is very prone to confirmation bias.

>Isn't this true for any skill

I'm not sure why it would hold that, if you do something for 40 hours a week, you'll necessarily improve if you do it for an even greater amount of time. And, besides, effectiveness as a developer is not just about coding anyway. A lot more goes into creating products and services than lines of code.

The underlying assumption is that when you code outside of work, you're working on things that are different from what you do at work.

That might be true for some physical/mechanical skills, such as sports. However, some quick googling shows that even professional athletes rarely average more than 40 hours a week practising over the course of a year, so even that's questionable.

And as for things like programming or any mental/intellectual skill like that? I highly doubt it.

As a general rule, you're going to see heavy diminishing returns on time invested. By the time you've spent 40 hours doing it, a few more hours on the weekend is unlikely to have any real benefit, and to the extent it causes burn out or takes the place of something that helps you stay healthy, it's likely to be a net negative.