My reaction would be that it is up to them to prove the devices are using I2C.
If there are known I2C devices in the circuit such as eeproms, then by using
them I have bought the implied license, even if interfaced to a "non-licensed"
device such as a PIC, where I have written a bit-banging program to do the
interface. If there are no known I2C devices in the circuit, the protocol is my
trade secret, until they can prove I have based it on I2C.