Xbox One Series X and family accounts

It actually would. There's an Xbox One, One S, and One X that are all in the same family. Then there is the Series S and Series X that are in a completely different family of consoles.

An Xbox One X is not the same as a Series X. The game will not work on an One X.

No such thing as an Xbox One Series X. The Xbox One family of consoles has an S and an X but are still just Xbox Ones. You need the Series family of consoles. The Xbox Series S or Xbox Series X. These are different from Xbox One S or Xbox One X.

The "One" and "Series" are the console types.

nope, most if not all current games on xbox one will be playable on the series x, eventually there will be games release just for the series x and they wont be playable on previouse console versions.

Xbox one s and Xbox one X are the same family, series x will be the new console and eventually (a couple of years down the line hopefully) games will only be release on the series x family so the xbox one x will not be able to play them.

If this is not confusing

the whole family from 2001

Xbox

Xbox 360

Xbox One

Xbox One S

Xbox One X

Then soon Xbox Series X and possibly Xbox Series S

Elaborating slightly: None of our family accounts on the same xbox aside from mine have xbox live gold (ultimate in my case). Previously on xbox one x, for instance, my wifes xbox account, she would sign into MWF3 360, go to live and join games. Now on
xbox series X, same exact steps, it throws the error. You could always play under other accounts as long as one account was a live paid account (360 or regular xbox one x titles), now it only works for one x or one x series x titles.