Henri Mulet (1878-1967) was a Parisian organist and composer. He didn't let not much music, because he destroyed all his manuscripts towards the end of the 1930's before retreating to southern France.
He is mainly known for his Esquisses Byzantines, whose Tu es Petra is the last piece. It could be described as the archetype of the "railway toccata" with alternated chords, like Widor's one is the archetype of the "sewing machine toccata". This piece is rarely played by French organists although it seems to be quite popular among Anglo-Saxon organists.
Mulet wrote essays about organ building in which he strongly advocates for mutations et mixtures, which were rather neglected in France at this period. He probably would have been pleased with the Steinmeyer which provides a good deal of both of them.
By the hazards of second hand books research, I bought an edition of his essay "Les tendances néfastes et anti-religieuses de l'orgue moderne" which belonged to the organ builder Rochesson which was contemporary of Mulet. We may suppose that Rochesson didn't completely agree with Mulet's statements because he wrote a manuscript comment in the book telling "Décidément, ce Mulet est un âne", a pun difficult to translate ; in English, Mulet = Mule and âne = donkey (and also âne = an ignoramus, in French slang).
BTW Ralph Downes selected Rochesson to provide the big reeds of the then new Royal Festival Hall organ.