It's not technically a ground but rather a bond between the coax and your home's electrical system. The idea is to equalize the electrical potential between the two systems, not protect from lightning strikes/power surges. If you want lightning protection, you need a 75 Ohm surge protector.
With that said, if the installation was not done correctly, Comcast will probably replace your modem for you. I'll ask an employee to help you file a damage claim and get that line installed correctly.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
I can help get the damage claim started for you. In order assist you, please send me a private message verifying your first and last name, street address including city state and zip code, and the full account number or phone number associated with your services.
To send a private message click on my name "ComcastChe", then click private message me.