One definition of "first" is "preceding all others" [0], so something with an index of 0 would still be the first item.
Apparently the definition of "zeroth" is "being numbered 0 in a series" [1], so something with an index of 0 could be both the "first" and the "zeroth" item...
However, another definition of "first" is "one that is number one in a series", so the headline of this post is both accurate and inaccurate, depending on which definition you want to use, I suppose.
Yea it's mostly a joke and shows how a lot of this is contextual. I personally consider 1999-2000 the transition into the 21st century. Arguments otherwise are little more than awkward pedantry. Year 2021 could be seen as either the 21 or 22 year of the 21st century but i'm willing to side with the validity of OP's title.
