There's a complete discussion of the evolution of ideas and the fact that the distinction between "androids" or "synthetic life" versus "robots" or "humanoids machines" arose after R.U.R.
But what makes us think R.U.R. is still about robots is that the play is explicit that the robots are assembled, not grown:
> His robots resemble more modern conceptions of man-made life forms, such as the Replicants in Blade Runner, the "hosts" in the Westworld TV series and the humanoid Cylons in the re-imagined Battlestar Galactica, but in Čapek's time there was no conception of modern genetic engineering (DNA's role in heredity was not confirmed until 1952). There are descriptions of kneading-troughs for robot skin, great vats for liver and brains, and a factory for producing bones. Nerve fibers, arteries, and intestines are spun on factory bobbins, while the robots themselves are assembled like automobiles. Čapek's robots are living biological beings, but they are still assembled, as opposed to grown or born.
In RUR the robots are closer, in design and manufacturing techniques, to the monster in Mary Shelley’s Frankenstein. The depiction chanted in the movies that show the monster as being built out of whole sections of dead bodies.
How about 1920: https://en.wikipedia.org/wiki/R.U.R.