This seems kinda concrete to me. And kinda jives with what Wikipedia has to say about a computing platform (though not neccisarily a web platform).
A computing platform or digital platform  is the environment in which a piece of software is executed. It may be the hardware or the operating system (OS), even a web browser and associated application programming interfaces, or other underlying software, as long as the program code is executed with it.
Though im wondering how useful of a term it actually is then the longer I think about it. Knowing something IS a platform doesn’t tell you if it’s hardware, software, an operating system, a desktop environment, a browser, a virtual machine, a programming language or a collection of services; any such nonsense.
Going by that relatively solid definition, “platform” is just digital context something can be said to run in. In that sense everything Turing complete can be said to be a platform. Yet I am hesitant to say Conway’s Game of Life is a platform in the same sense that Amazon’s Alexa is a platform.
We seem to USE the word for much more than what it’s strict deffinition implies.