Gods need worship is a pretty old trope, with the 13th starved of aether and everyone living there either dead or warped into voidsent it's likely their god, now deprived of worship and aether, would follow shortly after.
No idea how we'd know about that on the source though, considering the lack of divine intervention by any of them, and therefore have no real idea why our religion would be altered to reflect the lack of 13th god.