Yes, I think that because someone made a slip of the tongue – or had momentary brain fade – for whatever reason, that fact ALONE doesn't mean they are not 'up to the job' of Foreign Secretary. There may be other reasons why you might think he isn't. Mistakenly saying his wife is Japanese, not...
And yet they seemed to take it as an honest mistake – which he corrected almost immediately???
YES it was a huge mistake, but that alone DOESN'T mean he is 'clearly not up to the job', FFS!