To me it sounds like how the Stormfather used to describe himself in the past as "more storm than spren", because he didn't have a mind yet, no personality. He was just the concept of the storm created out of investiture. (Iirc he also describes himself in this time as more force than spren or something to that effect, but I can't remember exactly so don't quote me on it)
So by saying they bonded forces, the Sibling is saying that these bonds were so old that the bondsmith spren hadn't developed personalities yet, they were just forces.
