The key is the the second 'or' in: "in terms of Φ, or q, or it must be constant". You're correct about a constant M the flux is linear to charge and therefor th

The key is the the second 'or' in: "in terms of Φ, or q, or it must be constant". You're correct about a constant M the flux is linear to charge and therefor the device is a resistor. But it doesn't have to be. It's no longer a linear relationship and behaves differently than a resistor, resulting in a device similar but not the same. Thus why "ristor" is used in the name. To answer your question memristors cover the second case where the relationship isn't linear, thus tying the loose end resistors leave in the theory.