So, mechathun’s deathrattle works if he has reborn…but why?
It makes sense as deathrattles occur after a minion dies but conclude before the minion is reborn. So at the time of the deathrattle the condition is met.
Minion death > deathrattle check > minion reborn.
Another example is if you give say Tunnel Blaster reborn. You wouldn’t expect it’s deathrattle to kill the reborn version and it doesn’t.