Local Biomes within Larger Biome

I know some people refute that the biome helps determine the eggs you get from pokestops but I believe it to be true from my experiences.

I'm the opposite of you. No dratini's here and we always get Rhyhorns, Geodudes, Growlithe etc. So we rarely get any dratini eggs, even with the latest increase since we don't have water spawns here. There is one park near the river and that is the only place I have gotten my 2 dratini eggs from. My friends go down there as well to get eggs as that is the only chance for a dratini. My friend has gotten like 4-5 dratini eggs from that one area.

I've already hatched 3 larvitars and at least 4 Chanseys in the last month. But dratini's are so rare.

I have a very similar experience.

I live near the coast in a strong water biome (though I do not find many Dratini around and only have 1 x Dragonite). However, I have found a small area out in the dunes that also spawns various rock / ground types, where I can find some (not many) Rhyhorns, Geodudes, Digletts, Kabutos, etc and 1 x Larvitar.

This area also includes a Pokestop, which is my first spin every morning, and has given me provided me with a Larvitar, several Phanpy, Geodude and Rhyhorn eggs. I keep trying here for that elusive Aerodactyl.

I think that there is a chance eggs are affected by biome.

To be fair though Chansey were in a higher rarity tier prior to gen 2 eggs release. I was in a similar biome to your first one and still am....and have drawn 8 Chanseys since the update while having only pulled 6 the entire time prior to the change.

While I think biomes may affect egg draws in some small way I do not think your singular draw proves anything. Please do not read that as me not appreciating your addition to the information pool.

Thesliproad made a study and currently there's no evidence that supports that there's a correlation between biome and egg drops from pokestops. Spawns do vary from place to place depending on biome, but egg drops don't.

Link to the research:
https://thesilphroad.com/science/secret-egg-rarity-tiers-pokemon-go