I don't know much Japanese so I did some research. First of all I don't agree with what you say, everything would be Hyperbolic then. Every statement in fiction would be, the kanji
"電光石火" means
Lightning speed as you can see so what you say is wrong. "Lightning Speed" no one denies that it is used in a Hyperbolic way, but first of all you have to see the context behind everything. First of all, the
Databook is a book where the characteristics of the characters are explained in detail, as the
author himself specifies. Then if we go and see, the
Hyperbolas are not at all right to use in books, as they can be encyclopedias etc, and the
Databook is an encyclopedia.
Moving on, with your reasoning you would be telling me that every statement is Hyperbolic ignoring every possibility that there is in fiction. First of all you can't know what the author means, so since there are two basic possibilities let's start from 50% and now let's go into more detail. As I said before you would be ignoring every possibility that there is in fiction, "Lightning Speed" literally means speed of lightning. IRL is used to actually express a thing that goes very fast, but in fiction it is different. That is, what makes the expression "That car goes at the speed of lightning" Hyperbolic, basically it is the impossibility at a physical level that that car can go at MSH+. Now if we analyze in fiction, where characters have already shown
feats above the sound etc, there is no longer a physics of restriction, since that thing is already contradicted, therefore there is no longer the basic contradiction, which occurs in Hyperbole regarding speed, such as IRL. Because basically the Hyperbole is a contradiction, since by exaggerating, it goes to say things that contradict themselves. Now what I mean by this, I'm saying that since there are no contradictions, rather, supporting feats, i.e.
outspeed lightning and
they dodge them, there is even consistency. Also because it is logical that a
non-hashira, goes as fast as lightning, and an Hashira who is massively faster outspeeds lightning.