I quoted the manual to show that using dt within physics will have the opposite effect as using dt in other circumstances..
Using dt has the disadvantage when using physics that the same actions can give different results..
Exactly like what you said in your first post..
The fixed timerate that is build in in the physics behaviour makes the result predictable..
So using dt with physics will probably have the opposite effect you are aiming for, just like it says in the manual..
I couldn't explain it better myself, still can't, but if you'd rather read my words than the manual, there you have it..