Disclaimer: I don't really know where
The Chaotician's assumption stems from, but I'm by no means an expert on that topic, far from it!
All I can bring to the table in the face of the topic at hand is a list that fellow user
Mattapod shared with me a while back.
Sadly I irreversibly left that conversation a long time ago, but, luckily I had the foresight to copy-paste it into a Word document for future reference.
Please find it down below:
Pokémon obtainable by gift, in-game trade, purchase, fishing or stationary encounter:
Bulbasaur, Charmander, Squirtle, Pikachu, Nidoran (F), Nidorina, Nidorino, Vulpix, Clefairy, Jigglypuff, Parasect, Dugtrio, Abra, Machamp, Farfetch’d, Seel, Dewgong, Muk, Electrode, Hitmonlee, Hitmonchan, Lickitung, Rhydon, Tangela, Mr. Mime, Scyther, Jynx, Pinsir, Magikarp, Lapras, Eevee, Porygon, Omanyte, Kabuto, Aerodactyl, Snorlax, Articuno, Zapdos, Moltres, Dratini and Mewtwo