To understand their response, you have to understand the marketing of VFDs and the terms "Variable Torque" and "Constant Torque" as it applies to how they are sold.
The primary job of a motor is to supply torque, at a specific speed, which we relate to as "power". But still, the first part of that is the torque, because that is what the LOAD requires both to accomplish the work, and to accelerate the load from a standstill. But not all loads are created equal and with some, the amount of work required by the load varies greatly depending on the speed, we call those "Variable Torque" (VT) or "Quadratic" loads, because the power to speed relationship is a quadratic equation. As speed increases, the load requirement increases by the CUBE of the speed change. This also works vice versa of course, so as speed LOWERS, the load on the motor is lowered at the cube of the speed drop. Normally, meaning using full speed motors, we don't really care, it's just an oddity. But once we introduce a speed control method for the motor, such as a VFD, it makes a HUGE difference. So if I select a motor for full speed power, but run it at 50% speed, now the LOAD on that motor is reduced to 12.5% of what the original sizing was.
How that plays into VFD sizing is that from a marketing and pricing standpoint, VFDs are sized based upon the ability for the internal components to handle the maximum load, PLUS the ability to handle any temporary OVERLOAD conditions, including the extra torque that may be necessary to overcome inertia and accelerate it from a stop. But in that VT machine, like a centrifugal pump, the load inertia is very very low, plus the need for being able to handle an overload condition is usually very very slight, knowing that the pump will be sized for MAXIMUM flow at full speed. So there is no NEED for added overload capability of the VFD, it should never need it. Therefore the VFD industry began, years ago, to offer "down-sized" versions of their drives that are expected to ONLY be used on VT machines like centrifugal pumps and fans. The difference is, they CANNOT handle any overload conditions, but should never have to. So what you might call a 50HP VFD in all other applications can actually be used as a 60HP VFD if the load it strictly "Variable Torque".
Now enter your viscosity issue. The reality of the "never going to overload" issue is that, it only applies when all things are equal to the lab test conditions. When viscosity increases, meaning the specific gravity of the fluid increases, the "cube law" no longer applies as equally as it does with clean water. So that is the likely basis of there concern. The "60HP" rating of the drive is likely a VT rating, but they are concerned that with a higher specific gravity in the fluid, the actual power requirement curve of the motor may end up looking flatter than a simplistic centrifugal machine.
All that said however, your point about the pump motor having ALREADY been selected based on it being a slurry is likely valid as well. My guess is that they just do not want to "ASSume" that and the difference in price is not worth the risk of failure.
"Will work for (the memory of) salami"
