Have to say there is just no way watt per watt they are equal or something is very wrong.
@Ponky there is no doubt in my mind they may be seeing those results and your not the guy running around without a lot of knowledge just commenting without a good amount of thought and experience.
But it's absolutely impossible unless it's other limiting factors involved. You can produce more photons per watt my a huge margin with LED compared to HPS. even the old blurple lights imo will watt for watt out perform HPS. Now granted that's reaching a bit and may not be the case bit it would be surprising if watt to watt they don't.
There has been a shit ton of studies and peer documentation to show 100% they will outperform HPS. BUT... if you don't have the environment and overall grow conditions conducive to use more light then you absolutely may see no difference.
There are some differences needed when using LED you need to adjust for and the main one being temperature. Now in a perfect world we would all understand that air temps mean nothing and it's only plant Temps that matter and under LED you usually need to run significantly warmer.
In general plants will require more calcium, magnesium, potassium and a couple micros in higher ratios under LED. I have no definitive answer but I'm more than confident this is spectrum related ad this has a large impact on the morphology of the plant and type of growth. I will attribute it to the higher green and blue when compared to HPS.
There is so much more depth I can go into but let's see how the discussion plays out rather than my posts being a page long lol.