1 GHz allows for photon to move 3 m per cycle in vacuum. 10 GHz is 30 cm. Even less in fiber cable. I think that's a fundamental restriction of a size of an individual computing module. Of course you can stack modules in entire buildings just like you can stack cpus in servers in data center now.
They're talking about wavelengths ("per cycle"). But I'm not sure it makes more sense knowing that, since there's a fundamental disconnect between the signal frequency and the carrier frequency. I think QAM can even be used on a signal rate that's higher than the carrier frequency (as long as the carrier frequency is known), but I'm not 100% sure.
If we take our definition of "individual computing module" to be that it has a defined state during every tick of the clock, then there is a hard physical limit of 30cm for a module that runs at 10ghz. Anything larger must be operating asynchronously, as a distributed system.
Point is that interconnect between floor 1 and 5 might pose considerable challenges, thus greatly minimizing the potential advantages of having massive building sized computers
