If you look at the linked document, section GoBD konforme E-Mail Archivierung, you'll see that internal "archiving" functions are explicitly listed as not GoBD compliant. The goal, in my understanding, is to have an option to export emails in bulk and store them off-site.
Even in plain backup terms, the archive within the context of the service makes no provisions for redundancy and business continuity.
In addition to that, some businesses may be legally obliged to maintain business communication for certain period of time.
Currently in Tutanota, there is an option to export individual emails but I assume this is extremely tedious for users, especially businesses, with larger volumes of email messages.