(Not advice just my experience) Don't know about the ground rod, but I just plug my sheet in the extra ground in the house since it's neutral, never had any problems.
When I first learned about earthing, since electrons supposed to flow better through copper, I tried an experiment. I took a coated 12/2 copper wire, and it's not very big, stripped the coating two feet from one end and buried all three wires about one foot in the ground, ran it to my bed, stripped enough off and wound all three wires round and round to cover most of the bed, put sheets over it and slept on it, and was surprised by the positive result after a few days, so I bought two sheets, works for me. The ground rod that comes with the sheets for older homes that don't have the extra ground, are not copper nor very big at all and no instructions on any current measurements.
Supposedly, if you have static electricity build up in your body, just touch the ground with your bare hand, and the static will discharge as fast as lightening. So, any contact in any form with the ground should be fine. But, since I am getting electrons from the ground to me, I used that 12/2 copper wire. Maybe the electricians are thinking about measuring possible back-feed current, but I don't know I if that would relate to neutral only because no positive current is being used.
After you connect your ground wire to the wire that goes to your mat, you can take a volt meter, set it to continuity, stick one lead in the ground, the other lead in an extension cord, make contact with the cord to the plug that plugs to the sheet, if you have continuity it is grounded.
BTW, I was an affiliate with a company that sold earthing sheets. Mercola got his grounding idea from my website many years ago when I was on his forum, then, I was ousted (and not the only one) from VV because I gave a different opinion about one of his products.