I'm suspicious of the way they worded that, if it is depicted exactly as stated. Motor suppliers generally depict a turn down ratio, when discussing its use with a VFD, as a simple ratio without qualification as to torque. Indeed, one PURPOSE of a VFD is to MAINTAIN full torque at any speed, but within the motor's ability to keep itself cool. It is the turn down ratio that gives you that lower speed value, beyond which you must provide external cooling or shed load.
By qualifying the lowest speed saying "at minimum torque", they are implying that you have ALREADY been shedding load BEFORE you get to 210RPM. That's why I'm suspicious, it's almost as if they are being purposely cagey, dodging the question with obfuscation.
But, to give them the benefit of the doubt, you may have shared details with them that you left out here with us. For instance if the application is a centrifugal pump or fan and they knew it, then their response might be appropriate for the way they know it will be used. In a centrifugal machine, the load (torque) varies as the cube of the speed change. Generally, it's not necessary to specify a turn down ratio for a motor intended to be used on a centrifugal machine for this reason. So what they are saying is that at the point where your centrifugal load will require minimum torque and still perform useful work, their motor speed can be 210RPM, meaning it will work fine for you. This then may have simply been their own gentle way of telling you that you were asking for a specification that was meaningless in context.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
