Makes sense. Mylar bags are designed to be used with oxygen absorbers mostly. The humidity from the moist tobacco in the tins escaped and created humid conditions in the mylar bags that was conducive to rust. Throwing in an oxygen absorber would have prevented the issue.
If humidity escaped from the tin in the mylar bag, that same humidity would be escaping and drying out the tobacco if left unbagged. The seal on European style tins is not totally airtight.
If moisture is leaking out of the tin just leaving the tin outside will dry it out eventually. Rust can’t occur without oxygen so a sealed tin in a mylar bag with an oxygen absorbers would seem to be the best of both worlds.