Looks like most things I've read just assume it's only from the battlefield but the wording says all creatures. Does "all creatures" always only refer to active creatures because they're not creatures until in play, just creature cards?
“Creatures” (and “creature”) specifically means a creature permanent on the battlefield. So it will not affect creature cards anywhere else.
109.2. If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield.
609.2. Effects apply only to permanents unless the instruction’s text states otherwise or they clearly can apply only to objects in one or more other zones.
In order to refer to objects in other zones; an effect would specifically use the word “card” and specify the zone. See Birthing Pod for an example.
You are exactly right, a creature card is a 'card.'
A creature card being played, after mana has been paid and any other costs is a spell being cast.
A creature spell on the stack, before it resolves, is not a creature yet.
A player who has priority may cast a creature card from his or her hand during a main phase of his or her turn when the stack is empty. Casting a creature as a spell uses the stack. (See rule 601, “Casting Spells.”) 302.2.
Once it resolves, it is a permanent. A 'creature' permanent.
110.4. There are five permanent types: artifact, creature, enchantment, land, and planeswalker. Instant and sorcery cards can’t enter the battlefield and thus can’t be permanents. Some tribal cards can enter the battlefield and some can’t, depending on their other card types. See section 3, “Card Types.”
Once the spell resolves, ie everyone passed priority and let it resolve, it is now a 'creature' on the battlefield. These are the 'Creatures' that Hallowed Burial refers to.
Other answers have touched on this, but I think it needs to be more explicit: cards with the word "creature" on them are creature cards. They are not creatures. When they are cast, they summon a creature to the battlefield. The creature card is then used to represent the creature.
Cards and rules are sometimes loose about referring to a card and the entity it represents interchangeably, but there are never any creatures in the graveyard, only creature cards. You can see this in the wording of cards that bring things out of the graveyard, e.g.
Agadeem Occultist "Put target creature card from an opponent's graveyard onto the battlefield under your control if its converted mana cost is less than or equal to the number of Allies you control" [emphasis added]