The following spells [fireball, in this case] are added to the warlock spell list for you.
That's a plain statement that this warlock can cast fireball, once they are capable of casting third level spells. Spell level restrictions can be overcome when using a scroll with a successful spellcasting check at DC 10 + the spell's level, and so for fireball that is a spellcasting check at DC 13.
The relevant restriction dealing with spellcasting checks on scrolls is the spell level, not the caster's class level, and so with the Fiend Pact placing fireball on the warlock's spell list it becomes possible for that warlock to attempt to use a scroll of fireball.
In comments the question focused on a more specific case and on the wording of Clerics' domain spells feature:
Just to be completely clear, there's no equivalent limitation to the one clerics have, right? Cleric's Domain Spells section says: "Each domain has a list of spells [...] that you gain at the cleric levels noted in the domain description." I know it's hard to prove a negative, but a similar limitation was my main concern.
I'm not aware of any more specific rules which would supersede the general rules around this issue for clerics, so my ruling would default to the interpretation above.
In the same way that the example warlock has fireball on their spell list, but can't actually cast (or even learn) a 3rd-level spell yet, a cleric that has taken a domain gets access to more spells (even if they aren't "available" until later Cleric levels) - they are spells that can be learned by this class, which is what a class spell list represents. That the cleric definitely will learn those particular spells at a given level seems unrelated.
Crucially, a pact with a Fiend only places fireball on the spell list - it still has to be learned to actually be useable as a spell. That's pretty analogous to the cleric's situation, at least with regard to the mechanics that surround using a spell scroll.