The next milepost in the misnaming of church modes happened over a 75 to 100 year period ending in roughly 1675 ... when the church modes of Gregory were expressed as permutations of the then new major-minor scale system. That's when the modes became formalized into what we know and use today. The Greek names became convenient labels for particular scales, though there is no certain tie between the notes in any modern church mode and the notes in any ancient Greek mode. Locrian mode was 'invented' to complete a theoretical picture. It is unlikely that anyone ever actually sang anything in Locrian mode ... 'cept maybe for some jazzers.
Sometimes the church modes are described in terms of a process whereby the notes may be sounded. E.g., Phrygian mode can sounded by playing the white keys on a piano from E to E. But Phrygian mode is not just a C scale starting on a different note. Phrygian mode on E has E as a tonic. The church modes are in no way derived from a major scale. They were used for centuries before the major-minor scale system was developed.
I do tend to think of the modes as 'the major scale starting from a different point'; this is how they are often explained. So I'm intrigued by this piece - Is it true to say that the modes were thought of differently before some point when major-minor can be said to have become popular?