Both 10596 kHz USB and 10596.4 USB are known digital mode frequencies, and I have seen such a signal used to test data channels before. Note it is only 3 kHz wide from low to high tone (fitting inside the BW of most SSB data transmitters), and that tone steps are 500 Hz. HAARP is pretty much always wider than 3 kHz, except when it is sending dead carriers or OOK.
Also note that at that time there was no indicated signals on the HAARP waterfall.
I would strongly suspect test tones on a data channel.