You don't necessarily know what a long option means without looking it up, just because it is long. Firstly, if your native language isn't English, you may have to look it up in a dictionary. The ordinary meanings you find there may not reveal what that word means in the context of the program. So, at that point you're off to the documentation anyway.

I know what "hard" and "soft" are. Therefore, is it obvious what "git reset --soft" means? Hardly.

Once you know what it means, "--soft" jogs your memory by association a lot better than some "-X". So that would seem like it is less cognitive work. But when we have long options, it encourages the option vocabulary of a program to keep growing, which adds to the cognitive load.