The core philosophy of material design is to treat interface as a physical artefact, like pieces of paper overlaying each other for example. Everything from visual treatments to animations is derived from this singular philosophy.
However, as with any framework, it’s accompanied by technical spacing and layout specifications also. In a sense, many designers make the mistake of blindly adopting the more stylistic parts of the framework rather than the technical specifications, resulting in designs that essentially look like Google.
In the case of Dropbox, they’re app apparently works according to layout specifications, however it doesn’t subscribe to the core philosophy of Material Design in that nothing is actually material in the app, in fact it’s more flat than anything. In this sense, it’s not Material Design, except that it possibly abides by the physical layout specs according to Google.
I personally don’t subscribe to the core philosophy of Material Design, possibly because I’m a bit of a contrarian, but mainly because I don’t think the core philosophy of making interface material is valid, and in fact it will become less valid as time goes on.
Digital interfaces don’t adhere to physical constraints. They’re a tool to allow humans to interact with information. The end goal is for the interface to be seamless (your interpretation of what exactly that means is what makes this idea exciting). To some, it might mean no interface, to others it might be a replcation of the physical world.
I think Material Design is worth learning nonetheless, if not from a technical point of view. Their ideas on divisible base units of 4/8px and typographic rhythm are all great concepts.