To be clear I’m not doubting you, but how does this square with the fact that these are literal DVS-B tuners? Do they perform some decoding in hardware for this purpose?
The output of the dongle in it's normal DVB-T mode is an MPEG-2 Transport Stream. The maximum bitrate of DVB-T is 31.67 Mbps or ~4 Mbytes/s. It's thought that the USB interface was designed to support that rate and not a whole lot more (3.2 MS/s would be 6.4 Mbytes/s since it's 2 bytes per sample).
The key is that the general-purpose SDR mode is an originally-undocumented test/debug mode. Different drivers are used to access it; the standard DVB-T drivers don't provide access to it.
Is that true? RTL-SDR are derived from a chip designed for DVB-T tuners.