Except it is not unlimited, but instead undisclosed limit, discretionary. In other words, the contract is lying or you haven't read it.
Being fired over taking too much vacation time given the mentioned policy is potentially lawyer fodder.
They'd probably just fire you for 'not being a culture fit' or similar.
It's unlimited in the same way an "unlimited mobile data" plan with a 10GB soft-cap is.
Except unlimited has no real liability and 28 days has a potential value of at least 28 days of someone's salary. From an accounting standpoint it's a big deal.