Oscilloscope with 400 MHz but only 100 MS/s?
They're stating that the bandwidth of the analog front end is 400MHz.  For a periodic signal then you could fill in the waveform by sampling successive sweeps.
So you could for example see a 400Mhz sine wave on the scope (though 3dB down) with this technique.
For a single shot or non-periodic signal you would be limited by the sample rate.
From the manual: