This is an attitude more than anything I'd say, the "follow the calendar" mentality doesn't have to apply to you unless you choose so.
Taking longer (unplanned) amounts of time away from the keyboard is a different issue, though. If your work is truly isolated from the team and it's getting done on time, then it shouldn't matter when you're doing it. In fact, your employer wouldn't even know if you were at your desk or not as long as the work shows up.
In practice, though, most people's work isn't actually fully isolated and asynchronous. If other people need to work on something with you or ask questions, it's a huge pain to play ping-pong with messages back and forth every several hours or even every day to solve things that could have been accomplished in 5 minutes of synchronous chat.
This is why most remote companies will try to avoid creating teams that span too many timezones. Collaboration and teamwork plummet when real-time communication isn't a thing and nobody can ever predict when others will be available without scheduling meetings long in advance.
I know it's not popular on HN, but in my experience few engineering jobs are truly asynchronous and isolated like this. Flexing schedules here and there is honestly enough to do most things that people want and need to do on a regular basis anyway. It's the people who are disappearing for entire days (e.g. taking vacation days without actually declaring a vacation day) who cause the problems.
You have to be online on messaging services/contactable by email with quick response during normal hours, making it very hard to do much else in this time.
Covid has meant more people working remotely, but hasn't changed working schedules that much from what i have seen here at least.
We still live in a world full of companies that don’t allow software engineers to work in their best environment.
What's "online" with a "quick response" - suppose you step away for 45 mins to work out? Is someone giving you a hard time because your response time to an email or a Slack message is not immediate? Does that question or inquiry need to be handled that quickly?
Lots of things like this seem like a failure to set expectations. We're allowed to deliberately block our calendars, right? That might mean you close your email and do an hour of work solid, or work out, or run an errand. But it's possible, with some deliberate calendar planning, and frankly transparency with those expecting something from you.
"I won't be available for the next 45 mins because [X]" is a pretty easy statement to deliver
I can shift to free days which can even feel relaxing. But that means gifting my employer the best hours of my free days too.
When detached from the rhythm of the processes where you work together with others, it's easy to start slacking, or to start overworking, unless you have another source of rhythm. I've seen people critically dependent on the lunch break to have their lunch; without the external signal, they complained, they tend to forget about it, to the detriment of their health and mood.