I imagine both sorts of earlybirds exist, and I'd be interested in seeing how common each is.
I once spent a couple weeks living only by candlelight after 8:30 or so, no screens. It's hard not to get enough sleep that way. You can still read, play board games/cards, write, study, play or listen to music, prep/cook food, talk(!), and tons of other stuff, but go figure, those things don't tend to keep one from going to sleep at a reasonable hour (you may be surprised at what's a "reasonable hour" for you when literally far more total entertainment than was enjoyed over any entire given year in the Imperial capital of Rome isn't available at your fingertips).
Netflix, cable tv that's on 24/7 (not my thing, but lots of people use their TV that way), video games, news feeds, mindless browsing, "who was that one guy in that one thing? Hold on, I'll look it up", what's Trump up to now? (better check!), a few thousand lumens of artificial lighting after the sun goes down, and so on... you'll put off sleep for those, maybe without realizing it ("it's not like I even feel tired yet", sure, but you would if you hadn't been playing Playstation or watching Netflix or scrolling through Facebook in a bright room for 2 hours straight, and if you didn't have the option of doing the same or similar for another two hours)
Having a window to all the world, a vast library both greater and with a worse SNR than any that's ever existed, more high-quality, unique audio-visual entertainment options than existed in entire major cities a mere couple of decades ago (let alone 50 years ago), hundreds of immersive video games tuned for "engagement" at best and outright addiction at worst (remember when games would eventually get frustrating/boring enough, or you'd get stuck and have no immediate way of finding a solution, so after an hour or two you'd be sick of playing and just want to ride bikes or something? Those were the days.), a direct line to answer any stupid low-value question that pops in your head, near-instant communication with everyone you know, and so on (mixed in with tons of manipulative advertising and such), all in your house, all of the time, is maybe not healthy for most people.
It is my will being perpetually sated.
Pretend we are writing a fable in which a sorcerer always gets what he wants.
Consider what happens to a soul which always gets what it wants.
- Emily Bludworth de Barios
(excerpt, whole thing at http://www.forkliftohio.com/index.php?page=freight-31)
I generally come into the office around 10:30am, though sometimes it'll be 8am. I wouldn't mind coming in around 2pm and working into the evening, but I frequently need to work with other employees who have a more normal schedule, and I have an 11am standup so I have to be awake by then anyway. When I know I need to go to sleep, I pop a melatonin tablet and put on a sleep mask to block out light. When I first started, I forced myself to bed before 11 each night and made sure to be at the office before 9 each morning, until I observed that people didn't care about strict schedules.
There's an understanding that as long as I make the meetings I agree to (some are in person and require travel) and management is happy with my output, my schedule can be quite flexible. Some weeks will be less than 40 hours, some weeks will be > 50 because of some deep bug I just want to get over with. Some days I'll spend a bit too much time on HN or taking breaks to the snack kitchen, other days I'll sit down when I arrive in the morning and not stand again until late at night due to focus. It all tends to balance out. In any case when the lines aren't drawn clearly it's important to find out at what point the fuzzy boundary becomes solid. This "cultural shift" that OP was complaining about is just the new generation of employees finding out where the hard borders actually are, and discovering that at many firms they're a lot further back than they used to be and they can be pushed even further with a bit of pressure. I'm waiting to see someone show up to work in a fursuit, and wish there were an old-timer suited IBM employee there to witness it too. Many firms have realized that to a large extent, for many jobs what you wear simply doesn't matter.
At a previous job, I also had a pretty flexible schedule, but we tried to keep it "between 10 and 11" yet I kept coming in closer and closer to 11 all the time until a few times I was in after 11 and more than once I overslept and came in past noon. We changed it to "be here by 10 or we might have to cut ties". No further problems.