This question contains spoilers for the Lost Tomb of Khaem from the Out of the Abyss adventure, hence why this question's title is quite ambiguous as well. If you do not want this to be spoiled, avoid this question.
So, Dawnbringer (OotA, pg. 222, or pg. 58 in this free pdf) is a sentient Sun Blade (DMG, pg. 205) that is found during the Lost Tomb of Khaem mini-dungeon/side quest. The description of Dawnbringer (which uses similar wording to the Sun Blade) says:
... Dawnbringer appears to be a gilded longsword hilt. While grasping the hilt, you can use a bonus action to make a blade of pure radiance spring from the hilt, or cause the blade to disappear.
Both the Dawnbringer and the Sun Blade are listed as requiring attunement. The ability to make its blade appear or disappear requires attunement, since magic items that require attunement only allow you to use any of their magical properties without attuning to it first if mentioned explicitly in the description, and the description for the Dawnbringer/Sun Blade mention no such thing. Therefore, this is just a longsword hilt without a blade until you attune to it.
However, in the Lost Tomb of Khaem dungeon, this happens in the final room (OotA, pg. 38; or pg. 43 from the free pdf):
On initiative count 1 in the round in which Brysis [a wraith hostile to the party] attacks, the characters hear the telepathic voice that first called to them. “In the sarcophagus! I can help you!” See “Treasure” for more information.
So let's see the "Treasure" section below (same page):
The thin gold sheath covering Brysis’s sarcophagus can be pried loose and is worth 250 gp. Inside the stone sarcophagus, lying atop Brysis’s withered and mummified corpse, is a magic sword called Dawnbringer (see appendix B). This intelligent weapon is the source of the telepathic messages.
It would appear that Dawnbringer wishes to help you in the fight, hence "I can help you", but it's useless until you attune to it, which takes a lot longer than this fight would last. This leads me to believe one of two things:
a) Dawnbringer's blade was already "out" (however, this contradicts the description of the magic item in appendix B, which describes it as a longsword hilt, and nothing in the description of the room when the sword says "I can help you" implies otherwise, besides its intention to help you);
b) Dawnbringer instantly attunes to whomever grabs it, assuming they're not evil due to its attunement restrictions (this is not mentioned anywhere as being possible, but the Out of the Abyss adventure does set a precedent for it being possible to instantly attune to a sentient weapon if that's what it wants);
So, my question is, how can it be possible for Dawnbringer to help in the fight like it claims? Is there an option c) I've not thought of, or is option b) the correct answer (since I don't think option a) can be correct), and if option b) is correct, how so (since it doesn't explicitly mention anything like it does for the other example of instant attunement)?