Why do you think the marmora blades magically change form? What exactly triggers it? Keith's blade activated when he learned to quit (one of his biggest challenges) so does the knife magically sense when you somehow become marmora material™ by overcoming you challenges???? Explain!
*adjusts glasses* it’s space magic, my dude.
More specifically what I think is going on here is like… so quintessence is given two different descriptions in the show. In one sense it’s raw life energy, and that’s usually how we’ve heard it described.
But in the other we here allusions like Allura’s “The quintessence of the Lion is mirrored in his pilot.” And how specifically Altean energy is needed to make the wormholes work.
We also have the thing of how the Lions can sense their particular paladin, across the void of space, when said paladins aren’t carrying or wearing anything that transmits their location.
And more generally, the druid in s1e10 was able to do that funny owl thing and know right where Keith was- which we’ve also seen Keith do on several occasions, telling someone is there, or sensing Blue and Red.
So what I think is: quintessence is energy but there is basically a “soul frequency” to an individual person’s quintessence that contains information about them, and this kind of… broadcasts out of them.
And this can be picked up on, with varying levels of finesse.