20XX isn't a year. It's an ideal. A state of mind.

Every year from 2000 to 2099 can be part of 20xx. That's the point. It represents a range so that the year reference isn't dated. Like how back a few decades ago people would say IN THE YEAR TWO THOUSAND AND TEN like it was this impossible time. But we know it as ten years ago and it's not impressive now.

XX in roman numerals is 20.

XX in roman numerals is 20. Meaning you could write 2020 as 20XX, but technically in roman it would be MMXX