You have to remember the Japanese translation is well, for the Japanese audience. If it was Gaster I don't think it would make sense to hint at that with a similar typing style in English while not hinting at it whatsoever in Japanese. The translation should ideally be conveying the same information to both audiences. After all, if it was just him speaking more casually, wouldn't the english text not be in all caps to match?
Also, Ralsei says DELTA RUNE too. "This is the legend of DELTA RUNE." Notable too that both times Gaster said it on twitter it was DELTARUNE, one word, while Ralsei and this mystery guy both say DELTA RUNE, two words. I think DELTARUNE is the game and DELTA RUNE is... The story? The legend? Something ingame yet to be seen?
that is a good point and i do wonder why it isn't written like gaster's text normally is in japanese. i'm not really fluent enough to say if the speech patterns match, either.
from what i've seen on twitter, the japanese fans do seem to think it's gaster, too, so whether there's a similar speech pattern also present in the japanese version that i can't pick up on, or if they just got the info from the english version, or even if just the mysteriousness of the letter is enough to make them think that, the information is being conveyed somehow...
i wouldn't be super surprised if the valentine-guy turned out to be someone completely different, but i do think it would be odd to make a mysterious valentine with a very similar speaking pattern to gaster, plus several references to gaster, and have it not be at least some form of gaster, yknow?
as for the second point, i was specifically talking about the phrase "delta rune" referring to the real-life Video Game DELTARUNE, like in a meta context. ralsei references "the legend of DELTA RUNE" as an in-universe prophecy, but gaster and valentine-guy are referring to the actual game itself - note that valentine-guy immediately after says there was "a rumor of valentines day", which i interpreted as talking about fans speculating about the release date of chapter 3/4.
also, in the japanese version of the valentine it is actually written as "DELTARUNE" and not "DELTA RUNE", so i don't think the distinction is meaningful in this case!