The conversion method from dBm/MHz to dBm/500MHz is correct. -41.3 dBm/500MHz = -14.3 dBm/MHz.
I believe the -16dBm / 500Mz is just for illustration purposes. I think it likely originates from the TX power at the IC to achieve 14.3dB/MHz radiated power at the antenna with a typical 2dB gain antenna.
Note that the spectrum of the DW1000 isn’t perfectly “square”. In most regions, there is also bandwidth limitation of -10 dB for a 500MHz band. This might also result in a lower TX power figure if the bandwidth isn’t calibrated well.