Irrelevant for reasons I'll talk about later.
See below.
It's funny that you bring up authors retconning their work, because:
The "official" lore (which was written by MisterGameTheory and the TCT, not the developers, and is in itself riddled with plot holes and bad writing) was retconned itself a year after it released. Headcanon is
quite literally the only correct path now.
No it doesn't, have you looked at yknow, the actual structure of the bones and brain? Cthulhu having a brother is just logically one step: he has a brother. (something I'll elaborate more on later), meanwhile for skeletron and the eyes to actually be a part of cthulhu
they need to fit the body, and they don't. In fact they don't even fit
each other.
logically, Wouldn't that mean he brings back cthulhu if you
left him alone? Instead you interrupt the ritual, which means literally anything could happen as a result, from the player having a hallucination (which doesn't take any additional logic and is actually the simplest one, if you're gonna pull out ocam's razor), to the wrong eldritch being emerging. We know his
intent due to this passage but not the actual consequences of his actions.
Additionally, cthulhu actually
has TWO confirmed brothers in the lovecraftian lore, (K'thanid and Hastur) which closely resemble cthulhu, so it's not a logical leap to consider them at all. Hastur is strongly associated with the color yellow, and guess what color energy the mysterious tablet gives off? Guess what color the Ancient Vision is, which the cultist only summons after absorbing the energy from said tablet? You're taking the cultist's intent into consideration without considering that he could have simply made a mistake and grabbed the wrong tablet when preparing the ritual. In fact considering that actual
confirmed motifs of cthulhu are completely absent beyond the brain of cthulhu boss fight, it makes
more sense for the cultist to make such a mistake. We don't even know where the mysterious tablet
came from.
But then again, does any of this even matter? As shown above, headcanon is what the developers support, and there is no confirmation ingame that moon lord is cthulhu so the devs haven't contradicted that. In fact, there's just as much tying moon lord to Hastur as there is Cthulhu ingame, so there isn't a definitive way to know.
And honestly? I prefer this. Terraria in itself is a sandbox game, and by
not confirming the identity of moon lord as some known eldritch being with plenty of pre-established lore, you're allowed to fill it in yourself (A "lore sandbox")! Hell even in the Derplore: A Terrarian History, which is a headcanon I wrote, moon lord is not related to eldritch forces at all but was a powerful biological weapon that was constantly discarded and repurposed until it tried to lash out at all beings in vengeance.