Metal Gear 2 development history?

Hideo Kojima said development began after he came upon someone from the development team of Snake's Revenge on a train ride. So did this happen in 1989? Ads for Metal Gear 2 first appeared in December of that year, and Snake's Revenge was released in April 1990.

And assuming this is correct, it sounds to me like Metal Gear 2 was conceived first, if ads first appeared in December 1989.