I don't think thats true. You need to calculate equivalent f-stop for smaller sized sensors to compare the captured light. The f stop mentioned on the smartphone relates to the physical focal length while the 35mm-equivalent focal length is adjusted by the crop factor. As a result the sensor actually covers a small part of the area that could theoretically be lit with such a short focal length (light falling in from the side at large angles).
There is no equivalent f/stop.
There is an equivalent focal length but as I explained it has nothing to do with the amount of light captured by the photovoltaic elements.
The equivalent focal length is used only to infer the field of view.
Imagine a 14mm 2.8 on full frame - that's a very wide angle. Of course a 14mm 2.8 on a mobile phone size sensor will not be used a wide angle, as the senor will only cover a tiny amount of the arc behind the lens. While the two lenses theoretically "transport" the same amount of light, the sensor of the smart phone only sees a tiny amount of it in the center of the theoretical image circle. In practical terms of course the smaller-sensor lens will already be constructed to only have a very small image circle and therefore let through nowhere near as much light as the lens constructed for a larger sensor.
That's why it makes sense to consider equivalent f-stops for calculations of light amount passing to the sensor, it also has its place in calculation of DOF.
