I think the pre-requirement for legendary dogs is not simply the first 2 parts plus play time.
I don't have Ho-oh seen dex.
No Croconaw, Feraligatr, Porygon2 owned dex.
But I have the bell from Sicentist Rey and be able to fight guardians then catch a dog.
I just want to let you know that there is a mistake in the Carlho walkthrough.
You don't need to speak with the scientist in Bell tower to obtain the dog stone if you made the celebi quest.
This is two different quests and two different rewards.
The fact is that you need to have all caught and legendaries seen in pokedex to be able to catch Celebi, this why you need to do the dogs quest but you don't need to complete it to be able to caught Celebi because is a another quest. Bell tower scientist speak again is not needed for celebi quest.
Like I tell in an old message, the two quest are not a successive quests. Carl made the walktrought as a suite but it's not. It's two quest.