I got from both accounts that the Swedish doctor was being somewhat careless; he just presents it in a slightly better light - as one does.
I quite agree with you that Macdonald had no reason to make the story up and that it is probably, in essence, true but I think it is an important principle that statements appearing in a memoir written years after the event should not be treated as fact and repeated unless corroborated. There would be less bogus history around if the principle was observed.
There's no reason why the more authentic memoirs shouldn't be cited to give possible explanations of events, to add colour or to illustrate character, they just shouldn't be regarded as evidence without corroboration. I should point out that many of the negative views of Macdonald come from statements in Thiebault's memoirs being treated as fact and repeated without anyone checking them.