http://naruto.wikia.com/wiki/Hand_Seal
The writing implies you're right, the manga say anything for or against a half-seal, but the pictures of the full seals seems to imply that you can't do a full seal with one hand but you can get the same effect.
I wanna be an interpreter. It's legit. But only Deaf people can do it that fast as far as I know. I've been practicing for 5 years and still can't do it at that speed.
I think, just like any language, it also depends on how often you use it. Those who are Deaf probably speak it more often than those who aren't, and have (probably) signed since a young age, as such they are faster at it.
It is legit, and it is impressive, but it's not unique. Many Deaf individuals can sign the alphabet as well as other words and sentences as fast as this.
The writing implies you're right, the manga say anything for or against a half-seal, but the pictures of the full seals seems to imply that you can't do a full seal with one hand but you can get the same effect.