Morris does not believe this is a problem, as at least one of Jesus' promises was made before a general audience. That the priests so firmly remember the pledge is in distinct contrast to the disciples, who fled and are not mentioned as remembering it.
The original statement implies that Morris thinks there is no concern, as he believes that at least one of Jesus' vows was proclaimed in public, whereas the priests have a clear recollection of it, which contrasts with the disciples who ran away and are not mentioned as having any recollection.