^^ lionel, who's most definitely among said contributors, actually addresses this question in broader terms here:
They're actually not. rlang, however, is part of r-lib. I'm excerpting from an answer here to a different question (since the rest of it isn't particularly relevant) wherein I describe r-lib in further detail.
rlang is under active development and used for non-standard evaluation (NSE) in tidyverse packages, e.g. dplyr. You can learn more about that in the Programming with dplyr vignette:
It's also used in the in-progress second edition of Hadley's Advanced R book (below links to the section on metaprogramming): https://adv-r.hadley.nz/meta.html
I don't think that's necessarily true. As you've pointed out, neither depends on the other and sometimes when things are reworked, functionality from one package may migrate to another.