the 5hp/foot 'rule of thumb' for rotary mowers is a widely accepted common sense general guideline, not a sales gimmick.
it takes in consideration for overall implement weight, rotating mass, load % on the engine and powertrain, maneuvering, handling and stability, terrain, vegetation type and density, environmental conditions, etc.
regardless if it was derived from similar hp/foot guidelines for tillage implements, it's still a reasonable balance for
most machines for
most applications.
my family has been farming this area since the 50's. I started mowing 100+ acres in the early 80's. my folks also owned a commercial lawn mowing business for a few years while I was in high school. several large apartment complexes and hotels. I could weave a rope to the sun with number of blades of grass I've cut.
my 37 year career was large fleet heavy equipment maint/repair. 9600 pieces where I retired as a fleet service manager. hp and implement/attachment sizing guidelines are widely used across all industries.
bottom line is denying the limitations of a machine doesn't change it's capabilities. you can only run a machine at or above it's upper limits for so long until it leads to premature wear and failures. guidelines like these are your friend, embrace them.
as another old saying goes, ymmv.
good luck!
