I don’t know how they measure it now.
They have always been able to adjust the HP numbers either up or down according to whether it was to their advantage either way.
I had a 65 Olds Delta 88 with a 425 rocket V-8. It was rated at 360 HP but I suspect it was conservative as it ran like a bat out of hell even in that large heavy body.
Once upon a time I had a ‘70 Firebird Formula 400. Sucker cooked.