I think it's right on a line, and I see both sides of it. Most tables I've experienced including mine, waffle back and forth across that line when it comes to narration of the resolution of a proposition.
Part of the resolution of the narration of this proposition involved needing to make some sort of saving throw. If the save was successful it implies the character did something to mitigate against the damage. The narration that Maxperson is giving is implying that either the PC would take the fireball to the face or else they were forced to do something on reflex and instinct in order to avoid that, in this case hurl themselves behind a pillar.
Fundamentally, this is narration that I think is intended solely as color and not to make any meaningful change in the fictional positioning of the scene or to compromise the player's agency, and I think most players at most tables would take it at that.
But it does cross a bit over the line, and some tables - particularly ones sensitive to these sorts of concerns about player's right to self-narration - would hand it over to the player to narrate how they escaped the fireball contingent on the idea that the player could be trusted to narrate only as color and not try to gain some advantage in the fictional positioning out of the narration. This last bit is often a sticking point, and it's also worth noting that handing narration off to a player who is in the fog of war and so doesn't have all the information risks accidentally doing fictional positioning that isn't merely color. This is why for example, Matt Mercer - who has very good players whom he trusts a lot - nonetheless rarely hands narrative control of a scene over to a player fully except on a death blow or the conclusion of a scene.