Most cameras today have great image quality. That is to say, they do well on "measurable attributes" like dynamic range, sensor resolution, color accuracy, tonal reproduction, exposure accuracy and so forth. I also think it's true that "the camera doesn't matter" in the general sense, meaning I can use any camera to get great image quality these days. Not great images necessarily, as that's another thing entirely, but the image quality (those measurable attributes) should be pretty good. I always like to say that it's a great time to enjoy photography.
That said, I think the "the camera does matter" in the specific sense, to each individual. A landscape photographer I really like is Joe Cornish from the UK. He shoots both medium format (Phase One backs on a Linhof Technika) and 35mm (Sony A7r4 I believe). Can I hand him an iPhone and and have him give me a great image back? Probably, but he feels his best work is with those other cameras for specific deliberate reasons. The workflow and constraints those systems impose and the choices he is forced to make because of those things all play into how he thinks about, composes and delivers his images.
It's certainly true that someone else might be able to replicate an image with another camera, even an iPhone, after the fact, but I don't think that's the point. Taking the Phase / Technika combo as an example, it is a large, very heavy combo that's entirely manual focus and somewhat unforgiving. But the Rodenstock lenses used on those systems are exceptional and with this combo he can come back with a very high resolution, color-accurate, 16-18-stop dynamic range raw file (i.e. great IQ). Because of the constraints of weight, manual focus, need for deliberate framing using lens shift, tilt, swing will look different to what someone with a different set of decisions and constraints would make. The camera system (including lenses) can inform the final image.
Likewise, one may use an iPhone where you need to make a different set of decisions and work with another set of constraints that would inform the final image. Every system imposes some set of boundary conditions that require some form of decision making - even if unconscious - that shapes the image. It could even be something like ergonomics or menu systems that we gel with (or not). Or the lens. Or cost constraints. Or time...the list goes on. All of those play into the decision making process around an image that gets captured.
Back to my own "day job" of software development and architecture. My approach to how I do my work is different because I use a Mac than it would be if used a Windows PC or a Linux-based machine. I would get great "software quality" on any platform but the constraints imposed by using a Mac force decisions that help me - Ray the individual - get work done better (for me). It may be that "constraints" isn't really the right word because it carries negative connotations and isn't my goal. But fewer or no choices in one area drive creative or efficiency decisions elsewhere.
So "the camera doesn't matter" and "it's just a tool" are entirely correct but also equally correct to say that the choice you make in the tool you use can set up different pathways to getting to the same goal that can result in unique outcomes.
"Ray's Philosophy of the Day" - done!