Spoiler: they do in part, but very very slowly.
SharePoint Online Targeted Release tenants are starting to get the new Hub Sites that were promised last September.
Hub sites are a way to organize different sites together, without a site hierarchy. Hub sites are just ordinary sites, typically communication sites, that are designated as hubs. You can read about search, rollup and inherited theme elements elsewhere. For the purpose of MUI, what is important is that Hubs have an extra navigation menu, and this navigation menu is inherited by all the sites that are associated with the hub.
Sites that are associated with the Hub, let's call them Spokes, will get this extra navigation menu, but do not have the ability to edit this menu, only the Hub site can edit it.
The Hub navigation menu consists of two parts: the Hub site display name and the navigation links.