I don't buy that argument. Each platform has similar but different functions for doing all common things, the graphics API's are converging on a similar design

I don't buy that argument. Each platform has similar but different functions for doing all common things, the graphics API's are converging on a similar design as well. To make it clear, the main platforms for more AAA-like titles are Windows/PC, PS4, XboxOne, and now the Switch. The main platforms on mobile are iOS and Android. Each of these requires an adaptation layer to be written because they are all different to each other, some more than others though. So you can "lock" yourself into one of these platforms, "lock" yourself into using an engine or existing library which covers some subset of those platforms, or write your own adaptation layer and engine. For learning and experimenting it's quite easy and convenient to focus on a particular platform, Windows/PC in this case is a good simple default since it's available and doesn't require NDAs, licensing, etc.