We know that it's entirely possible to make artificial voidsent through means outside of what created the Void, so it stands to reason that the opposite absolutely creates Sin Eaters. And if there's any civilization that learned that through the classic 'fuck around>find out' pipeline, it was the one that invented White Magic. No connection to the First required.

Not that them connecting to the First probably would've produced anything; the wibbly-wobbly nature of time in relation to the shards seems to only relate to the fact they all flow in the same direction but different rates. If it was possible for them to be flowing backward, G'raha's plan would look very different, as would the Ascians' in relation to the Thirteenth. So if Amdapor found the First, they were more likely to have found Ronkans than Sin Eaters.