The only place I recall where Schuon refers to Sikhism is in Understanding Islam.
Having said (not surprisingly) that no new revelation is possible after a “certain cyclical period and the hardening of the terrestrial ambiance”, the Middle Ages marking “gross modo the final limit” and Islam therefore being “the last world religion”, he adds in a footnote:
“As for the Sikh brotherhood, this is an esoterism analogous to that of Kabir, the special position of which is explained by the quite exceptional conditions arising from the contiguity of Hinduism and Sufism; but here too it is a case of a final possibility” (pp. 47-48; p. 48, note 13).
As you rightly surmised, Schuon felt obliged to acknowledge the validity of Sikhism: not as a religion in its own right, however, but rather as a very “exceptional” esoteric synthesis of two other traditions, and as such the last of its kind.
I would not call it, as you did, a “grudging” acceptance, but it was certainly a qualified acceptance, one meant to forestall any attempt to turn this exception into a precedent.