As an experiment, I disabled editing on a different Parcel Type and tried the same edit.
I received the "Layer is not editable..." error. I think that this is because Pro wanted to adjust the other Parcel Type along with my selected parcel since they are topologically connected.
To counter this, I turned off topology, and I was then able to edit the selected feature without the error.
To me this isn't a great fix, because you'd want to have topology on in most cases, but I don't know a great way to counter this if you need to prevent a layer from being modified.