The videogame is from the '80s; it became iconic partially because it allowed players to choose the music to listen to "in the car", all of them synths (obviously), and one of the songs you could choose was called "Splash Wave".

Folks like Kavinsky later (re)popularized synths and made explicit throwbacks to the game, like the album name. I don't know where the term "Synthwave" came from, but it does look like one of those throwbacks.

Synthwave name is more likely to be related various "wave" genres, like Darkwave or Coldwave.