It's not an appreciable issue. If somebody has physical access to a a miners server rack, it's game over no matter what "memory encryption" is used. As for the attack specifically, I doubt it's ever been used outside of security demonstrations and high level infiltration rather than something as completely insignificant as Bitcoin mining.

This kind of attack would be hard to pull of due to the nature of Bitcoin and if you are capable of accessing the physical machine why not just steal it and mine with it?

It's hard because what is there to steal? You can steal the solution to a new block but you will have to work very quickly. As soon as a miner finds a solution there are only a few milliseconds before the block is hammered out and sent out to the network. When it reaches a node it can be considered a "game over".

You'd have to act fast, say you detected the machine sending a completed block and managed to make sure the block never propagated beyond the machine. You'd be racing against the Bitcoin network, they will find a solution if you don't. It's possible that miners will find a solution within minutes.

You could possibly disconnect it's connection to the internet but that will only disrupt the pool the physical machine is most likely participating in. It won't be able to complete it's own share and that's the end of it.