Guardians can be revived. But it's very clear there were Guardians during the height of the Traveler. The Grimoire Cards clearly state we are revived Guardians. So we were all ready Guardians. What we do not know is the origin point for Guardians. And the strong implication in the cinematic, of three astronauts, imply through visual cues that these were the First Guardians.

I didn't read all of the Grimoire cards tbh. The thing is, if we were already guardians, wouldn't we already know about the traveler? Why would it have to be explained to us again?

We did know about the Traveler, we only asked what happened, meaning we fell before the Traveler did. Only the city and the Traveler's fate are delivered to us as direct exposition. Even our character never asks about "Guardians". Implying we know the term. Now, whether there were actual named "Guardians" or just those blessed by the light is up for debate. We know the Traveler chose and blessed those with the Light, and I'm simply presenting evidence through visual cues that these astronauts were the first blessed by the Light, making them the First Guardians. What was their fate? Have they been revived? Did they witness the Collapse? These are all important questions that follow, especially if they play a role in the narrative. But I don't feel like Bungie would have specifically shown and pointed out three individuals if there wasn't context or narrative drive for it.