Slowing the Earth’s rotation on its axis would both reduce the number of days in the year (because the day would be slightly longer) and lengthen the synodic month (because it would take slightly longer for the Earth to ‘catch up’ with the Moon); this percentage is an interesting number, as the ancient year was for quite a while exactly 12 months of 30 days each.
You’re correct, I don’t know why I recalled the Sumerian account of a supposed second moon near collision as slowing the earth’s rotation of the sun. It didn’t, it was the cause attributed for an increase in the speed of the earth’s rotation upon its own axis, leading to the 365+ day year, instead of the symmetrical, 12 month, 360 day year of ancient calendars.
“Faster” days could at least in part account for the extraordinary longevity of men in certain ancient accounts as well.