At the end of the day, you can overthink these things. Does it make it easier? Yes ...... but so what, just do it longer or faster than you would do outside.
If you come out of the gym, gasping for air, yelling 'Im dying!' ..... then your body doesnt care whether you were on a belt or on solid ground for the previous hour.
People can be a bit prissy about treadmills, but the bottom line is,the human body is a pretty simple machine, and it doesnt sit there and go 'Oh well, I see they did all this on a treadmill so Im going to reduce my improvement by 10% as a penalty.'
If you bust a gut at the gym, thats all that matters.