Doesn't that allow for Stuart Armstrong to have a false belief?
"Stuart Armstrong cannot justify a belief in this sentence" might be a stronger form since it implicitly refers to any logical or rational reasoning that might be used to justify the belief.
There are some obvious weaknesses to human Godel sentences. Stuart Armstrong is not a permanent name. Additionally our biology changes over time so we probably can't have permanent personal Godel sentences.
Building on the very bad Gödel anti-AI argument (computers's are formal and can't prove their own Gödel sentence, hence no AI), it occurred to me that you could make a strong case that humans could never recognise a human Gödel sentence. The argument goes like this:
Now, the more usual way of dealing with human Gödel sentences is to say that humans are inconsistent, but that the inconsistency doesn't blow up our reasoning system because we use something akin to relevance logic.
But, if we do assume humans are consistent (or can become consistent), then it does seem we will never knowingly encounter our own Gödel sentences. As to where this G could hide and we could never find it? My guess would be somewhere in the larger ordinals, up where our understanding starts to get flaky.