You should have read the whole page you linked. ^^
Basically, there is a difference between a decade and the xth decade (and the same applies to centuries, milleniums, etc).
A decade is any period of 10 years, with whatever starting point you want. You can say “I’ve been working on this for a decade” without needing to have started on a year ending with 1.
The xth decade is the xth decade from the starting point of the calendar which is the year one. So the first decade is 1-10, the second is 11-20, etc.
The 90s is used as a short way to say “the decade of the years starting with 199x” so mean 1990-1999. It’s ten years so it’s a decade. But it’s not the 200th decade (1991-2000).