The confusion started with the Eggstravaganza 2017 (April 13 through April 20 2017) event that enabled starter hatches from Eggs collected during the event.
After the event passed, any eggs collected during it still had a chance to hatch a Johto Starter, which lead to even more confusion.
Moreover, there is quite a lot of confusion about how eggs and egg hatches work. Let’s clear up some things:
Before (and after) Eggstravaganza 2017 it was (and still is) not possible to hatch Generation II starters. If you have leftover eggs that you picked up during that event, they can hatch starters. Otherwise, no.
And that’s it — Generation 2 (Johto) starters can not hatch from regular 5KM eggs, only eggs picked up during Eggstravaganza 2017 (April 13 through April 20 2017)!