Just to point out something some are missing: just because a spren says they were the only one that came - that doesn't mean there is only one in Roshar now. They go through memory loss trauma as part of crossing over - so anything they say about the spren tactics might be a bit suspect - but more importantly, its out of date. Syl might just be the first honorspren to disobey. Wyndle could have been only the first gardener the ring sent. That info was only good at the point they were last in Shadesmar.
Therefore, we cannot assume Ym / Stump are truthwatchers based on what Wyndle thinks about his status. We know he can't see another radiant's spren. It would be really difficult to know if there was another of his type unless they deliberately revealed themselves.
