Can you unlock a Mac that was locked from Find my iPhone & prevent it from being erased?
According to the literature, what should have happened is that the erase command should have removed the PIN and the message you sent.  Instead, they shouldn't appear and the system should start wiping the drive immediately.  Looks like it didn't get the erase command or wasn't online to receive the command.  I'd AVOID turning it on anywhere it could get online.  Contact Apple support or take it in.  If you have a Time Machine backup, you could try the PIN and restore if necessary.