This is correct. Highlights are not local changes, they are globals changes that are synced to iCloud for use on all your devices. Since they are not saved in the database, they are used for Live caches. In the same way you can also highlight and online cache and it will be highlighted offline.
Yes, you can do this. When viewing an offline list, tap the ... button on the top right and choose Highlight > Remove Highlight > All Caches. You can also go to the More tab then Highlighting where you can remove all highlights from everywhere in the app.