- What do they mean? Are it a kind of phrasing marks?
- Were they added by the editor?
The Misa de Notre Dame by de Machaut is an other example :
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Maybe these scores you present are transcriptions of mensural notation, which allowed for a special notation for ligatures in vocal music. Brackets above the notes are used to represent ligatures in transcriptions of mensural notation to modern notation. You may find many details on the transcription of mensural notation to modern notation under the link provided above.
While the second example you provide confirms my assumption immediately, the first example with the additional slur markings in the soprano confuses me still. Taking a look at the original shows that it's not even in modal notation (and there's not indication of ligatures). Therefore, I would hazard a guess and suggest that those (implicit) ligatures were added by the editor.