Methos drinking milk in the show is especially likely if the show was originally made in another country with different ideas about what's appropriate for kids to watch. Maybe the Japanese saw this as a great opportunity, both to educate their kids and to export this show with customized episodes for other countries' histories. Drinking alcohol is pretty common in Japanese anime, even the ones aimed at kids, so when the show was brought to the US, they replaced the beer with milk. And all these kids grow up and get a real shock when they actually read Methos's journals.

Also, in all the confusion caused by the release of Methos's journals, I have to wonder what role the fake Methos will play. Sure, he's dead, but a number of Immortals have met him as Methos so that should at least cause some more confusion about Methos's appearance. Many of the ticked-off Immortals will be looking for the fake one instead, especially if they think that he was only pretending to be nice and pacifistic.