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Neither. It's because mons that hatch from 2km eggs have a 1km buddy distance.
As a general rule (there are a couple of exceptions):
Pokemon from 2km eggs = 1km buddy distance
Pokemon from 5km eggs = 3km buddy distance
Pokemon from 7km eggs = 3km buddy distance
Pokemon from 10km eggs = 5km buddy distance
Legendaries = 20km buddy distance
This is definitely not the case, even in the beginning. Gen 1 starters originally hatched from 2k eggs, but they are 3k buddies. This may have been the only exception at first, but at this point many 5k buddies have moved out of 10k eggs (Pinsir, Scyther, Magmar, Electabuzz, Hitmons, etc.) While many gen 1 and 2 commons don't hatch at all (Pidgey, Paras, Sentret, Ledyba, etc.)