On 6.1, I've noticed that admin group users can edit a page which is locked by workflow lock payload process without unlocking that page although the user who started the workflow cannot edit that page while being locked by lock payload process. Is it a correct behavior?
When a page is locked either manually or programmatically. The user (or object) that put the lock on the page can break the lock. However, if the user is not available or the object forgets to unlock the page, only the "admin" account can unlock any page by opening the page and clicking on "Unlock Page" from the page properties drop down
NOTE: The original user holding the lock does have permission to unlock the page. However, other users, even if a member of the administrators group, DO NOT have the right to unlock pages that have been locked. Only the "admin" account has this right. See  for more details.
Admin are users , with R/W access to the entire repo. So other than the users, only admin can edit on locked pages.
Yes, Only the admin or the person who locked the page can edit or unlock the page.