It’s not reading the contents of RAM via EM emanations, it’s using the EM emanations caused by certain memory access patterns as a side channel to exfiltrate data. Of course, that data could be anything, including whatever is in RAM, but the point is that you need to be running the code that generates the necessary memory access patterns to transmit the bits of data. This is not like TEMPEST where you can reconstruct a video display just using the emanations.