In this case it's not just a library like the wrappers for tensorflow already present in many languages (including python), it's something like a Swift language fork in which the language constructs are analyzed by the forked compiler during compilation in order to directly generate an equivalent tensorflow execution graph (without manually creating a tape through library calls). Thus it's a Swift (compiler plugin) for tensorflow.

The intention was to merge the fork into the main language making Swift differentiation tools first class within the language.

I had the same question for the naming of Windows Subsystem for Linux (WSL), which IMO should be named "Linux Subsystem for Windows"

The best explanation I can think of is that the people naming it wanted the first word to emphasize the thing that's most important to them. In Microsoft's case, that's of course Windows. Perhaps the same is true for the S4TF folks.