"Back then", the only 10 speed bikes were a 2x5.
As the years went by, they added more cogs on the back. Now up to 12 or 13?
So, a 24 speed could be a 3x8 or a 2x12. That's 2 entirely different chain widths. (chains & cogs get narrower as the number of speeds increases)
30+ years later, we just go by the number of cogs in back when referring to chain width to avoid the confusion.
See-
https://bike.bikegremlin.com/3555/bi...ion-standards/

Scroll down to the 2nd chart to show how cogs are narrower as the number of speeds increase.
https://sheldonbrown.com/cribsheet-spacing.html