We've decided to advertise the fact that HackRF One operates all the way down to 1 MHz, not just to 10 MHz. This isn't a change to the hardware design; it is simply an acknowledgment that the hardware has always worked at such low frequencies and that we support operation down to 1 MHz.
In fact, HackRF One can even function below 1 MHz, but the performance drops considerably as the frequency decreases. The curve is reasonably flat down to about 1 MHz, so we consider that to be the lower limit for most uses.
Now that we've seen consistent low frequency performance across multiple manufacturing runs, we're comfortable changing the official specification: HackRF One operates from 1 MHz to 6 GHz. Try attaching a long wire antenna to listen to shortwave radio!
Although HackRF One has reasonable performance down to 1 MHz, it performs better at higher frequencies. To get the best possible performance down to 1 MHz and lower, I recommend using an external upconverter/downconverter such as the excellent Ham It Up, open source hardware designed by Opendous.