Not sure of their rational. I do know it was designed specifically for rubbers, polypropylene and the like. Maybe the way it bonds or fails to dry to other fabrics (they explicitly state for it to work properly it needs to be buffered by hand, which may not work as well with fabrics).
Their website clearly states in bold
Not for use on unfinished leathers (such as suede), fabrics (canvas) or floorings.
Might be worth a call. Might be of an issue of not reaching full design/benefit versus actually causing harm.