the answer must be in why does it return so many results with libc++, the same as with just libc.
yaourt libc++abi does give a result, but libc++ gives the same with pacopts
So I suspect you found a bug in pacopts, try yaourt :)
% pacopts aur -i "libc++abi"
info :: libc++abi
Repository : aur
Name : libc++abi
PackageBase : libc++
Version : 6.0.0-1
URL : https://libcxx.llvm.org/
AUR Page : https://aur.archlinux.org/packages/libc++abi
Depends On : gcc-libs
Makedepends : clang cmake ninja python libunwind
License : MIT custom:University of Illinois/NCSA Open Source License
Votes : 156
Popularity : 18.65
Out of Date : No
Maintainer : WoefulDerelict
Submitted : Sat 04 Feb 2017 06:09:37 PM EET
Last Modified : Wed 21 Mar 2018 06:20:57 AM EET
Description : Low level support for the LLVM C++ standard library.