There has been some anecdotal evidence here that it holds up a reboot. I am thinking of one person in particular that had gone over a year with no porn but kept masturbating, but he never shook the symptoms of porn addiction, and had trouble finding evidence of rewiring.
My sense is that if porn and masturbation are intrinsically linked in your brain, then using one will stimulate the pathways of the other. The more the pathways are stimulated, the less you'll be able to rewire. I'm not convinced that a porn addict can pleasure himself without any porn pathways firing up. Think of all the hours and brain chemicals that went into associating the two actions together in your brain. That is what makes it different from other things that have dopamine hits. The hormone rush of masturbation reinforces the porn, and the porn images create a desire to masturbate. I'm sure if you had a milkshake with each porn session, having a milkshake during reboot would become a problem too, though that scenario requires a bit more abstract imagination! To have a porn-less masturbation session would take so much dissociation from porn, that it would make me question whether there is a porn addiction there to begin with. I could be wrong... some people claim opposite experience, but it is foreign to everything I learned during my reboot.
Whether you want to consider it a relapse or not, that's up to you. Setting goals and evaluating those goals is all in your court. Personally, I think dwelling on streaks and relapses causes undue discouragement, and when we're discouraged we just relapse more. I think it would be reasonable to consider it something detrimental to a reboot, something you don't want to repeat, but also not a relapse.
I'm an advocate of hard mode. But, hard mode doesn't need to come all at once. Someone trying to lose weight isn't necessarily going to start all of the dietary changes and caloric restrictions on day one, but may dismantle old habits one by one on a path to permanent healthy eating habits. If that is the reboot path for you, then that can be okay. I'd be looking for ways to eventually get to hard mode though, especially if you want to see the real benefits of rebooting.