Note that Mythbusters video was revisited later in the series and the claims reversed: https://mythresults.com/episode38

It's not an equivalent question either, the question is intensity not sum.

Very cool, need to check that out.

However, isn't intensity equal to sum when it comes to light? At least in the visible spectrum, more energy just shifts the wavelength...perceived intensity comes from photon count.

Photons sent on a 5 year near c journey would arrive over e.g. 5 days instead of 5 years, hence the intensity. I.e. it doesn't take 5 years for the light you emitted 5 light days ago to arrive but the light you emitted 5 years ago may only be 5 light day ahead.