I’ve never heard anyone say “F stab” for fstab.
I wonder how many other people code-switch in this way to try make themselves better understood, when actually we would still understand one another if we just used our internal pronunciation?
I suspect that split pronunciation between head and mouth is very common. I do the same thing with some words, like giblet, which in my head has a hard g, but when I say it I say it with a soft g.