Yes, you need that focus time. But running into someone on another team can be very valuable - you might learn that they're solving the same problem, you might learn that they're consuming the same API you're trying to figure out, you might learn that they're planning on working with your team in a few weeks, etc.
I would argue the benefits of having more of your state in Slack more than make up for “surprise productive watercooler meeting” thing that managers are so hot on. I think I’ve had that interaction maybe twice in 25+ years, and one of those times was just pointless gossip in the end.
That's a good point. I would go so far as to say that the majority of what engineers do is communicate with their future selves and other engineers, in that writing software is to use a communication medium. What code you write today will need to be read and understood later. What the code does on the computer is only a part of software.
Out of all the companies out there, I'd imagine Apple benefits from this the least. With all the badge-locked doors, literal curtains hiding projects from one another, and internal NDAs preventing one engineer from even disclosing what they are working on with another engineer, cross-pollination of ideas is going to be almost impossible. "Serendipitous conversations" is not something that comes to mind when one thinks about Apple's internal engineering culture.