That's correct, the US has 0 legal holidays, everything is contractual. That typically means around 7-8 days and builds up with seniority.
It's pretty poor: https://www.bls.gov/news.release/ebs.t05.htm
But anything contractual for the most in-demand jobs (such as software engineers today), is way better than the minimum legal standard or the average contractual standard. From pay to insurance to holidays to things like housing, free food, sports and mental counseling.
The basic rule is that you're better off in a more unequal society when you're at the top, and better off in a more equal society when you're at the bottom. There's of course a limit to this logic.
Then if you take the software devs that tend to frequent HN, (the majority of devs at a FAANG firm will know of HN and visit it from some to time), you're looking at say the top quartile among software devs. It's quite clear that most software devs are better off in the US in many respects, and that their holiday perks can be way better than the average American or European. Plenty of great software companies in the US offer unlimited time off. Most people don't really use it though, it's more a cultural thing.
I think working conditions relative to the USA have gotten significantly worse in the U.K. for a lot of professional roles over the past 20 years though - the pay offered in many U.K. SWE roles is often risible and the generally anaemic economy since the credit crunch arguably hasn’t helped.
At other places, more competitive perhaps, they may be right to evaluate that a dip in work output is noticed and nobody is taking into account the time off.
Anyway, without judging why, it appears to be a fact that at some places people take less time off when unlimited vacation policies are brought in.
It's great if you feel you can actually take it, but most people don't.
It generally means there isn't a defined amount or minimum amount, and everyone knows if you actually take a lot of time off, your output will not look as good compared with your colleages.
The undertone of competing with colleages for recognition hasn't gone away after all. Even if you're not ambitious, you probably value job security, so you don't want to look like a slacker or low output person.
So in practice, at "unlimited vacation" companies people tend to take less vacation than they do at companies with a defined amount of vacation.
Local culture also ends up influencing what people do. I'm working for a company that has "unlimited vacation" and also a very international, fully remote workforce. What they have found is, even though the company policy is the same everywhere, people in the USA choose to take much less official vacation (up to about 2 weeks) than people in France (about 5 weeks). This is the individual's choice remember. Also the amount people have taken has reduced during the pandemic.
They are actually trying to get people to take more vacation, and have set a minimum amount to try to enforce this. My guess is people don't want to be seen as slacking during a time when alternative employment prospects are uncertain and keeping remote work is a priority.