That assumes the encrypted content is transferred in plaintext (not over https). I think you'd be hard pressed to find a HTTPS client that decrypts on the GPU. And then of course, the thing being decrypted needs to be big enough for the process to take long enough for them to perform the attack, which means many gigabytes (terabytes?). And then you'd still need to send the data from the mitm server back to the browser for processing before you start measuring anything.

It's obvious they accomplished something, but the scenario they set up simply isn't something that is practical in the real world

DRM-encrypted streaming video, perhaps? Gigabytes of encrypted data that needs to be decrypted and displayed?

But you've still got to shuttle that data to the browser and it has to be decrypted on the GPU. I think you'd be hard pressed to find that anywhere in the wild.

Modern CPUs can decrypt AES with specialized instructions at a rate that's practically measured in GB/s. Especially for streaming video that doesn't need to decrypt the full stream at once (and where you're probably already using the GPU to, you know, render the video) why would you use the GPU for AES?