As far as I can tell, that the parent was ‘Géant des Batailles’ has been from the beginning a widely-held assumption by fanciers rather than a stated fact from Trouillard or some other authoritative person. Standish, in his ads introducing it, does not make the claim.

Gardeners' Chronicle p. 167-168 (Feb. 11, 1888)

SOME NEGLECTED ROSES.
HYBRID PERPETUALS.
"Wild Rose"

Eugéne Appert.—Well do I remember the sensation Mr. Standish created when be brought up a stand of this flower to the National show held many years ago at Hanover Square Rooms. "Have you seen Standish's new Rose?" was heard on every sides and yet now it is very much relegated to the background. It was raised at Angers, by Trouillard, Lefroy's foreman, and is evidently of the Géant des Batailles race—that race which once reigned supreme; it is of a deep velvety-crimson colour, unlike any other Rose that I know; it is, however, defective in shape, and has, as the "Giant" and most of his family, a tendency to mildew.