Be wary, there was no scam attempt. They failed to credit a valid tx due to a bug in mastercoin-explorer.com and said it was up to me to deal with this issue.
The issue has been dealt with and fixed thanks to Tachikoma, yet they still won't credit my msc. If someone knew about this bug before me (or any other mastercoin-explorer.com bug, because apparently they only rely on this website for valid/invalid tx), and successfully withdrew funds, they might be insolvent and are holding my funds hostage blaming me.

Until this issue is solved, I would be wary of depositing funds into it.