Just with a look, you can discern that this statement is not stated in a matter-of-fact way. This is not an elaborate explanation of the attack that informs us what it does or what it's made of. The purpose of this statement is to tell a character that he is going to die at the hand of Mask, with a "beam of light" phrase added without further context.
He doesn't need to put glasses and a suit on, then give you a 50 minute lecture, accompanied by a 20 page thesis on how his attack works.
If I have a sword and call it a sword, are you going to question if it's a sword because I didn't give you a molecular breakdown and a history of it's creation?
The statement is in fact made in a "matter-of-fact" way. Mask 100% intends on massacring and brutalizing Renji until Renji is dead. He displays clear intent to kill with his actions and statement, then proceeds to inform the reader that his attack is a "beam of light".
I agree that "beam of light" needs further context before making a conclusive decision, but let's see what else you say first.
More importantly, let's take a look at Mask's character, since he is the person speaking. Something to know about Mask is he has a habit of adding flair to his language. He calls his attacks "punches and headbutts of justice", he refers to himself as a "star", that his "soul is burning up with justice". All of these are hyperboles.
First, what part of "beam of light" sounds hyperbolic to you? If I say "die villainous scum at the explosion of my nuclear bomb!", is nuclear bomb hyperbolic?
Second, "of justice" is not inherently hyperbolic either. To Mask and the rest of the Quincy, he is on the side of Justice, to Mask he is the good guy and the Shinigami are the bad guy. The fact he uses "childish" terms is a non-point. Every time he refers to himself as "justice" he is being literal, as he believes himself just. Every time he refers to the enemy as "villains" he is being literal, as he believes them to be evil. Every time he refers to himself as the "star" (which star doesn't mean celestial body star here, it means star as in hero, star of the show, etc) he is being literal, as he believes himself to be the hero who will eradicate the villainy (Shinigami) from this world.
So, no in actuality he is not being hyperbolic at all. This is merely a case of Mask acting "cartoony", using a "child-like" vocabulary, acting like your classic comic book hero of when your parent's parents were kids.
Let's look at the statement again after taking the into account that Mask has a habit of adding flair and using hyperboles: "The villain shall die by a hero's beam of light".
The way this statement is delivered, it can easily be interpreted as something symbolic. Like villains, who are linked with darkness, shall perish at the hands of a hero, who spread light. All the talk about "justice this and justice that", this flowery language perfectly fits Mask. You can see the similarities between "headbutt of justice" and "hero's beam of light". Both justice and light are spread by the heroes in the world.
Considering Mask's statements are not inherently hyperbolic by definition, because hyperboles by definition are not literal, and Mask himself is very literal, this is just a faulty interpretation. Allow me to re-propose a better interpretation backed up by the feats of the attack itself.
anyway for new comers:
光線/Kōsen beam; light ray
The beam is called light
Reflecting in a reflective material (Zanpakutô already demonstrated to be reflective )
Travel straight
Does not explode, simply burns / vaporizes what comes in contact
vsbattles.fandom.com
Calc itself is already accepted at
1.29c
As Cyber so graciously points out with the scans, Mask fires a laser that is refered to as a "beam of light", said "beam of light" later reflects off the provably reflective surface of a blade.
https://vsbattles.com/threads/light-speed-qualifications.120346/post-3961835 As AKM agreed to within the most recent LS CRT, "unless contradicted, beam of light, satisfies the fourth requirement, given it is made from a reliable source."
https://vsbattles.com/threads/bleach-renji-and-mask.120729/post-3975629 As Elizhaa and many others share the sentiment of, Mask being the creator of the technique is a reliable source.
Considering the technique creator calls it a "beam of light", it reflects like light, it has no contradictions to being light speed, it is light speed.
