You are correct in the road world, 11 speed cassettes got wider and thus a longer freehub and 10 speed cassettes would need a spacer. In the MTB world, 11 and 12 speed HG cassettes have the large cogs pinned together and the large cogs kind of sit over the inside end of the freehub, so it packs 11 or 12 cogs in the same 8/9/10 speed freehub. Now that road groupsets are coming with wider range cassettes it gets a bit foggy, because the 11-28 to 11-32 cassettes are road, and 11-42+ are MTB (even if they come in a road groupset), not sure about mid range like an 11-36.