Note: Depending on the size of the mat being installed, a dedicated circuit without GFCI protection may be required. To disconnect the GFCI protected outlet, call a certified electrician.
At least they have built-in ELCI, right?
They may be referring simply to capacitive losses- a mat of this design is, by nature, going to shunt some current straight to ground by simple capacitance and may nuisance-trip the GFCI.
Every 15A or 20A outlet pretty much anywhere a consumer would be putting a cord-connected mat like this would have GFCI protection as absolutely required by NEC. The only exception would be an inaccessible dedicated outlet specifically for this mat, which is where the sparky comes in. Of course, they don't qualify this, and is joe homeowner REALLY going to pay to have a new dedicated outlet installed, or just defeat the GFCI he's got his extension cord plugged into when it starts tripping?