I think that although contrary to external appearances people are never singly defined - we all have different psychologies and facets, some that we consciously or unconsciously embrace or expound in given situations others that we consciously or unconsciously repress or try to tame, without needing to be (innately) something extreme or unhealthy - that's why I like that aspect of his work, it just runs with it.
He also says "so that my qualities of inhibition and rational thought are suspended", so he basically thinks of "inhibition" and "rational thought" as valuable (and/or higher value than other qualities, probably given his usual day to day work and education) - but his actual "work" - the thing by which he is remembered and valued - seems to be an attempt to break from those "qualities".
I think you've just sketched Pessoa's theory of creativity.
It's really wrinkling my brain to think of an author's character being a "better" writer than the author himself. Although, wrinkles aside, I suppose this effect could be achieved via the editing process; edit the "character's" writings with more time and care than one's own writing.