Is it a flight violation to mishear ATC instructions for exiting the runway?
I don't think you violated any regulations. It was an ATC instruction, not an ATC clearance. 

It was an instruction you were unable to comply with. The instruction was badly timed and was obviously not clear since you did not correctly understand it.  In the busy time during the landing roll, you understood something different, and read back what you understood it to be. 

The landing roll-out can be a busy time needing the attention of both pilots.  ATC issuing an instruction while you are basically still in the landing roll should not be expected to be complied with unless it is happens to be convenient for you to do so.   

Many airlines SOP say that you do nothing until you are clear of the runway. Both pilots remain heads up and don't touch anything. The pilot in command alone will decide where it is safe to tun off the runway. If ATC makes a "Request", and it can be safely complied with, then the PIC may decide to deviate from SOP and comply. 

If you ran off the side of the runway trying to comply with an ATC "request", who do you think will be held responsible? (I'll give you a hint, it won't be ATC)

This is a prime example of why ATC should not be doing this.