"The best camera is the one you have with you" is the version I'm most familiar with, but it basically means the same thing. Most of us photography enthusiasts are willing to carry different amounts of kit depending on where we are going and what we want to photograph. If I'm walking on one of our local beaches and see a bald eagle high up in a tree that I want to photograph, my iPhone will be useless. So will a DSLR with a wide angle lens. What you really want is a 300mm lens at a minimum. Of course now you're talking big shoulder bag or backpack for camera gear. I'm not going to carry that if I'm just walking to work and want to have a camera on me for unexpected photo opportunities. It's also not the end of the world if I don't get that photo of a bald eagle. Plenty of bald eagle photos in the world, but if I enjoy photographing eagles (which I do) then there will be times when it is worth it to me to haul the gear.
I'm probably an extreme example, but I have several combinations of kit that range from pocket-sized (just my iPhone) to coat-pocket sized (my Olympus OM-D with a pancake prime) to small shoulder bag (OM-D with several lenses) and on up to full kit backpack (two camera bodies with lenses ranging from ultrawide to long tele and a tripod). I choose what combination of gear to take with me based on where I'm going, what I will be doing, and what I hope to photograph. Even though I have some excellent highly specialized photography equipment, I'm super excited about the photographic capabilities of the iPhone X. What photographer wouldn't be excited about having a better camera that fits easily into a jean pocket? That said, there will still be plenty of situations where I will prefer to use my other camera gear because of what I like to photograph and what kind of results I expect to be able to achieve.
Sean