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?
Depending on the size of the mat that is being installed, a dedicated circuit without GFCI protection may be required.
Some of the mats are 240 volts due to their large size. The exterior GFCI requirement from 210.8(A) and (B) only applies to 125 volt, 15 and 20 amp circuits. Although it is not required for larger circuits, the property owner needs to decide the level of liability they are willing to take with the mat, especially if the mat is going to be in a public area. I presume these mats are tough but not bullet proof. With shoe cleats, high heals, and just general traffic will eventually damage it. Although it has ELCI protection, it is not the same as GFCI protection. ELCI has a higher tripping point. If a 240 volt mat were to be used, I would consider using a GFCI breaker especially if there is anything conductive within hands reach where the mat was going to be installed and the level of relaibility in that the mat will be properly inspected.