The scale-method of writing Gregorian chant had just been invented and was still in the formative stages.
In researching the origin of Gregorian chant, I learned something that I have repeatedly heard in the Maronite Church.
The Gregorian chant is the oldest musical manifestation of the Occident and it has its roots in the songs of the old synagogues, since Jesus Christ`s times.source ref.
Both draw their inspiration from the early christian church.
I understand that Hildegard's music was essentially liturgical (in other words it wasn't "art music" or "performance music," it was intended for sacred/church worship usage) and yet some of it is very challenging: for instance it spans 2 1/2 octaves, which is beyond the vocal range most people can sing readily. Some think most of the choir nuns may have been singing a single tone in the background (called a drone).
I keep hoping they'll find more of her manuscripts. Some of her vast correspondance --- plus that of many others --- were destroyed in the conflagrations of WWII :o(
War kills more than the present. It kills the past.