That’s right. The Processing compiler (whether the IDE command line) concatenates the sketch tabs into one file. The folder name is the sketch name, and the corresponding tab name (which must match) is the first tab concatenated is the first file concatonated – the entrypoint, like “main()”.
You can test this by creating a named sketch Test, leaving the first tab Test.pde entirely empty, and creating a second tab Tab.pde with code: println("hello world");
The sketch compiles and runs – it requires that same-named pde file, but doesn’t require anything in it. The same applies to draw(), setup(), et cetera – you may put them all in Tab.pde, so long as the required same-named pde file exists.