I believe the "High Density" term was coined when AC was still operating 9-abreast 777 in the XM config.

The Swiss ntu was the first batch of 77W with 10-abreast seating and Vantage business class seat, there were indeed much higher density than XM 777.

Then they replaced the vantage seats, ordered a few additional fins in the same config and refurbished the old XM cabin into today's 10-abreast layout. I suppose seat count per square foot is still lower than the "HD" 77W, but the density of seat in each cabin in the same square footage are identical. I suppose high/low J is the more accurate description of the two cabin layouts.