@AKM
Then that makes sense, at first glance, but then you'd have to assume the character (who, for the sake of the example, has confirmed light speed attacks) isn't trying to turn it at the fastest attack speed they have, and/or they don't have control over the attack's speed when actively manipulating it, which needs to be confirmed to be true rather than assumed needlessly. We shouldn't assume a character's attack speed as a result of the substance they use is SOLELY because of the substance they use, because that ADDS a limitation to a character that isn't explicitly stated.
Example explaination: Ah'Tur, a light manipulator; "The only reason my attacks are light speed is because i'm using the substance 'light', which moves at around 1c, to attack. I can't actually make my attacks go that speed."
We don't add weaknesses that aren't there, just as we don't add strengths. That clarification would need to be there explicitly, otherwise we're using headcanon to draw limits about a character's abilities when we lack information on their mechanics to make claims this precise. We usually assume a character has control over their power when they... control it... To arbitrarily claim that this character isn't normally in full control of the speed of their attack with nothing more than a guess is exactly what you accuse us of doing it seems.
Things might be different if we have the character's ability confirmed to operate as light in many aspects, is called light, but is never explicitly stated to move at light speed though...?
Even then, like I said, you'd be more adding that variable to the situation (one that's likely not accounted for) than taking the most simple inferences. It also, again, assumes that they're not responsible for the speed of their attack when it's linear due to its nature— in this case the power being light in the first place would make it light speed against— or rather irrelevant to their will.
Which needs to be stated, otherwise we assume that they're choosing to push it as fast as they are firing it- since the alternative seems to be a more unique mechanic of a power that needs to be clarified rather than actually typical of any form of elemental manipulation— again, adding a mechanic, trait, or weakness to the character without reason. We wouldn't do the same with other powers.
Fire, energy/Ki, and earth manipulators are able to control the movement and speed of their substances according to their will. Even if electricity and light typically have defined speeds, why would we assume that their users have no native control over the movement and speed of those substances because of that when that's not at all consistent in fiction enough for us to even consider it and blatantly goes against what the power is supposed to be, as you stated that Light Manipulators would have this ability, meaning we all agree on what substance manipulation can do?
Let alone consistent with either your nor any others' understandings of superpowers. By this logic, Fire manipulators can't ever control what their fire burns, or water manipulators can't ever keep things dry even after being hit with water, as these powers also have innate traits like these— although these cases at least need to be proven positive to be true, as again, we try our best not to assume things without at least a "good enough" reason to. Percy Jackson would like to have a word with you about the latter, as he's swam in water without even getting wet due to his water manipulation.
It would be best if we ignored that then...? Not because you don't have any basis in science, but because, besides adding things with little ability to comment on one way or the other and assuming they exist with no reason, this small hang up would be the one thing to destroy every other conclusive piece of evidence— say it's light in every way but the bending and has no speed statement— would we say it's not real light then, even though it's been directly stated to be and functions to be so? I disagree with that assessment, especially when we're looking for extremely specific variables that are hard to come by in most contexts and making the assumption that there's a difference between the speed of their substance (light or electricity) being the product of their power itself or that speed being something they choose— or some unique mechanic of the power that explains away the bending...
I dislike the idea if this is gonna be used to delegitimize hard light substance statements and/or override the functions that make it opperate as light, as we can neither gain this information reliably, nor should we assume this information is relevant given how all other superpowers work and are treated, and it's one point vs 3 pieces in favor of it.
If it's stated point blank period to be light EXPLICITLY, it's probably light, and doubly if it does light things like reflecting off of mirrors. I'm of the opinion these standards are best used when the light statements are vague and/or lack context to make the light based statements viable, or when there are no light explanations in general and all we see are blasts of shiny things.