That’s still a metaphor; how processes actually occupy space isn’t relevant to the distinction. If the backend is running on a laptop right in front of you, it’s still a backend. The frontend is probably on the same machine, nether in front of or behind the backend “really.” But we see the frontend’s display, just like we see things that are in front of other things (and not things that are behind other things,) thus the metaphor.

I agree that it is a metaphor. I meant to reply to the claim that there was no semantic connection, but I agree that I chose wording that unfortunately seemed to contrast literal truth with metaphorical truth.