You and XKCD are actually wrong, but not for the reason you are probably assuming.

https://en.wikipedia.org/wiki/Decade

A decade is any 10 year period. It could be 1983 though 1992 or it could be the more convenient 1980 to 1989 commonly known as "the 80s", or it could be 1981 to 1990.

https://www.dictionary.com/browse/decade

decade (noun):

1. a period of ten years:
   the three decades from 1776 to 1806.
2. a period of ten years beginning with a year whose last digit is zero: the decade of the 1980s.
3. a group, set, or series of ten.

You are not wrong in saying a new decade is between 2020 and 2029. You are wrong for claiming they are wrong when they say a new decade is between 2021 to 2030. You are both correct in the sense they are both new decades. Every year is arguably a new decade.

I think many of you are assuming "decade" is confined to specific periods such as "the 80s" or "the 90s". If you are talking about 2020-2029, then you should say "the 20s".