This means that there was some who did not currently believe. Does not seem to exclude the idea of those who do not currently believe from having believed at an earlier time.
“Did I not choose you, the twelve, and one of you is a devil?”
Similarly, this means one of the twelve is currently a devil. Does not say that they always were.
Look, I am not sure the Bible makes clear whether or not Judas "believed" at one point, but even in Jesus's own parable of the seeds the word was planted in some places that received it gladly but when the hard times came things changed. And also, the versus cited here simply do not establish that Judas never believed.