You should read that article. They list different bulbs. Par has the same problem with diffusion there an apogee formula now addressing it. The two half way decent lux apps use the camera. It says so.A. The apps dont use the camera. They use the light sensor that the software then uses for the camera ISO auto adjustment.
B. "The olden days"is also a time period when almost all reef aquarists were using similar bulbs, either incandecent or hallide 6500K lighting so saying 3000 lux at the sandbed was an easily comparable statement tabk to tank. Todays technology simply makes LUX and nearly useless measurement as even fixture to fixture some quality defects, tank particulate, even salt concentration can effect LUX levels. Let alone as previously stated many times, the simple lower end of spectrum where lux diminishes and PAR/PUR rockets upward in measureable values.