She's saying it's obsolete since you don't have a level 2 castle due to reset. Since mith is used by the upgrade itself.
In general, don't reset when you've got the upgrade unless you're willing to spend another 4 billion gold coins.
Thanks Jaceton. The 5 free mithril were handed out just as a gift from the devs. Since you reset, you lost your castle upgrade, and also lose all your mithril. It is not a permanent item, and so you will lose it when you reset.
I just got told by the Mods that you are meant to in fact keep mithril when you reset, and that they ARE a permanent item. If you lost them, then it is a bug. Email support@athinkingape.com and let them know you lost them. Or even contact them through feedback.
A developer just told the moderators that it goes away with the reset. (Prevents 0 stats having large bonuses probably. Who knows of the reasons. It just works this way.) . So yeah, it goes with the reset .. . @MM - you were right for a period .