>any span with the same start and end point must have length zero

"a year" is not a point, it's a span. so is a decade.

>Decade 1 is year(s) 10 through 19

but the property you are trying to describe goes hand in hand with 0-based indexing, and thus we see that ISO Decade 0 is >sad tuba< 9 years long

It is actually 10 years long by how ISO 8601 defines a decade.

Also it's worth noting here that ISO 8601 has a year zero, which equates to 1BC in historical terms.