That would be between solar cycles and lunar cycles: seems enticing. That would make Methuselah 80yo when he reportedly died; but there are problems still as Seth (for example) became a father at 105, which if it were months would be 8¾ years.

Another possibility is that when it says Methuselah it just means "Methuselah's blood line" or "Methuselah the tribe"; it's a Hebraic thing (elsewhere in the OT) to refer to a tribe as if referring to the founding person.

Or, the person writing down the numbers used a different system, through error or otherwise.