The sample rate is approximately 1MHz, so the highest frequency it can reliably display is 500Hz (look at Nyquist Rate). Even then, the displayed wave won’t be square.
Chances are the output frequency is affected by the MCU also having to update the display among other things, so at higher frequencies I’d expect it to be a bit off. Even if it’s driven by a timer interrupt, it’s likely that screen updating will have a higher priority.
I’m looking forward to finishing my degree and having time to mess around with the firmware!