A sufficiently motivated researcher can, in fact, use a smart phone as a desktop PC. The results might be "sort of" reasonable.
But such experiments simply point out that behavior is, and ought to be, different on devices with different form factors, user interfaces, screen sizes and software loads.
To use an analogy, in the history of media, we always tend to begin using a new medium on the pattern of the older medium. So, when "stage" was the dominant mass performance medium, and the movie business began, what did movies look like?
The experience was pretty much the same as a live performance, only captured on film. In fact, the new medium arguably was less rich, as an experience, since the "audio track" was missing.
In the realm of devices, we likely have a tendency to envision "how to use" a new category of devices through the prism of older devices we already are familiar with, as well.
The point is not so much that some common experiences (using email or messaging) will be common to most or all of the platforms. Beyond that, we will eventually discover that each device category creates a distinctive "highest and best use," despite the common features.