That's a trick called equivalent input sampling. Basically shifts the sample clock a little bit around while looking at the same waveform. The catch is it only works on repetitive waves, and you are still limited by the analog frontend's rise time.

The fastest scope I know of is a 256 GSPS done by interleaved ADC's. Analog bandwidth is 100 GHz Here's a recent submission on the bad boy:

https://news.ycombinator.com/item?id=31774734