True. But in this case, "making sh*t up" is the only way to resolve the issue, since there is no rule to tie the "extra attack" feature to attacks granted by spells.
Hence the reason why we are discussing ray of frost with such passion.
I mean, it clearly states you make "a" ranged spell attack against the target. When twin spell adds another target, it does not change "a", which means singular. So how can that work?
A frigid beam of blue-white light streaks toward a creature within range. Make a ranged spell attack against the target. On a hit, it takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn.
So, you get 2 targets (which is what twinspell does), and nothing else changes (rules do what they say, no more): so only 1 ("a") ranged spell attack. There is no rule to tie "extra attacks" to spells. So you can twin spell "ray of frost", which lets you pick which one of the targets you can make your one ranged spell attack on. And it also say "A frigid beam" - again, singular. If there are two targets hit, how can one beam effect both?
I am sure there are some people who expoit the rules and make naughty word up, like twin spell changes the text from "make a ranged spell attack" to "make ranged spell attacks on each target". Or that instead of one ray, there are two rays. DMs should make up their own mind about this kind of thing, but some admirble DMs will stop that kind of exploit cold. DMs without a brain in their heads may use JC's tweets to excuse them from thinking about it, and JC might say it is permitted.
Not everyone feels a need to play D&D with exploits like that, and not every DM tolerates it. When you bend the rules to get an effect -- like ray of frost producing two beams -- it is an exploit, regardless of the effect.
Now if the sorcerer had some special ability -- like, a feat that let them point at two people at once using one hand -- then I could seen permitting this.