You know well that I tend to believe Macdonald's version where I can and I am quite happy to believe this particular story, he had no reason to make it up. However I do hold that an unsupported story told over 15 years later is not evidence. If he was telling the story shortly afterwards (and I'd bet he was!) when it could have been contradicted then there is a stronger presumption in its favour but his Souvenirs weren't intended for publication. So Hans-Karl is quite correct in considering the story doubtful.