I tend to believe too that this may be 'likely' but as you say there is no proof as far as we know. Both Rousselot and H Knotel were very serious researchers who must be treated with respect. However, they may not have had any more original sources available than we do on this particular subject. Without wishing to denigrate their valuable contribution, when considering their output it should be noted that both used earlier but non-contempory material including for instance the possibly suspect Marckholsheim manuscript.