The 50GHz or 20ps figure is actually the limit of the gear the scientists had available - from the paper at https://iopscience.iop.org/article/10.1088/1361-6463/ad9284/...

"the signal state transitions were established at less than 20 ps, which is comparable to the bandwidth limit of the oscilloscope utilized at 50 GHz. [...] the initial rise time constant may be shorter than the measurement limits of the oscilloscope."

Sure. But it’s on them to prove their claim that the device is faster and they haven’t done that. There are faster scopes and you can rent them.