Same thing goes for a 20-minute max effort or even an hour etc.
Atm I am somewhat unfit (relative to my past/potential) so when I do a round on the pads in muay thai/boxing which could be 2-4 minutes, my heart rate sits around 180-185 for the duration of the round. Often, sparring is actually more relaxed than pads because the coaches like to work us hard.
I have noticed that if my training meaningfully reaches 173+ I will usually feel like a vegetable for several hours after, usually the whole day. I get endorphins and heightened awareness around the 150-172 region.
"Max effort" is kind of meaningless because it means different things to different people - when I was 16 my swimming coach pushed me with sprints until I puked, not THAT uncommon amongst competitive athletes and I was a little out of shape that one time. I rarely see people push themselves that hard during HIIT classes, myself included...
It’s just that some people have a max that is lower than others, or at some points in your life your effort is lower than at others, as dictated by physical, mental, or psychological capacity for that person at that moment.
For example a normally sedentary person might find it mentally and psychologically uncomfortable to exercise strenuously. That doesn’t mean they’re not putting forth max effort.
Max effort means max effort.