Is this bannable? I got reported for...

[Pseudopsia]

Is it against the rules to not respond to a tell from a GM? (honest question, I don't know if it is or not)

The GMs have other means of detecting botters than whether or not they respond to tells.

Thought you're reading TOS. It's in there:

In addition to the License Limitations, there are other activities that you are forbidden from doing in connection with the Game ("Prohibited Activities"). In its discretion, Square Enix may use a variety of methods to deal with violations of this User Agreement and the Prohibited Activities, including, but not limited to, any of the following:

• Issuing a warning;
• Placing a character in a "virtual jail" for a specified period of time;

You don't have to respond with a tell, but you better be able to stop what you're doing.