Pretty much all of us in Europe take about 6 weeks paid leave every year and it's not up for negotiation.

Those companies with unlimited time off should enforce you taking the legal minimum. They don't always.

If you take too much time off, in the UK you can be fired. No unfair dismissal if you've worked less than 2 years (what is the average tenure of a software developer, again?).

Spelling it out - UK workers get a minimum of 28 days leave but that includes mandated national holidays which are few in that country. Employment law is weak, and working conditions poor. Without the salary of the US, people frequently don't take their annual leave allocation.

It depends on the company people work for.

Which is the topic of the article.