Well as i was re-reading this serie in English (as i don't own the book anymore but that's not the point), i thought that i could explain the whole "the marmot puts the chocolate in the aluminium paper".
The translation is indeed correct, it's a reference to a very well known Milka advertisement that probably aired some 15-20 years ago (for multiples years in different ways after that), since then it's not uncommon for people to use the sentence to talk about something they don't believe is possible and even the brand itself has been referencing it in newer advertisement.
If you're interested just check youtube for "pub milka marmotte" and you should be able to find it