Being on-call while you do not get called upon is 24 / 7 work because you have to live your entire life around being available.
Like the blog post mentions, grocery shopping has implications because if you happen to have an on-call event while shopping it means leaving your cart to run to your car to address the situation because you can't leave your house without your work laptop.
It means never going to the beach alone on a Saturday because while you could bring your laptop with you, if you go swimming and there's an on-call event you can't address it because you have no way to get notified while you're swimming in the ocean.
It also means going to the movies with an expectation that if you get called 15 minutes into the movie you're leaving. Likewise, if you're mid-date and get called you're out of luck.
It means never enjoying being able to walk around while being disconnected from the world. It means if you're at your mom's funeral giving a eulogy you leave mid-speech to address PagerDuty.
Then there's knowing at any given second your phone can notify you of an event and you have to put the volume at maximum and place it right next to your face every night with an expectation that you could be woken up at any second.