One question about the PDOA hardware

I saw in the AOA measurement document that the spacing between the two antennas should be half wavelength or less(I calculated about 15mm), but on the official hardware I measured the PCB files that the spacing between the antennas was 20mm.
Why is it longer than half a wavelength?

To avoid phase wrapping, we use an antenna separation of 0.45 Lamda. We consider the central frequency of Channel 5 wich is 6.5 GHz.