Conan not respecting compiler.libcxx abi setting - c++

Question

I have the following conan profile:
$ conan profile show default
Configuration for profile default:
[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=gcc
compiler.version=11
compiler.cppstd=20
compiler.libcxx=libstdc++
build_type=Release
[options]
[conf]
[build_requires]
[env]
Note that compiler.libcxx is the old ABI (libstdc++) and not the new ABI (libstdc++11). This is intentional. Also I'm specifying cppstd=20.
I then made a custom conanfile for {fmt}, which tries to package up fmt 9.0. That looks like, in its entirety:
from conans import ConanFile, CMake, tools
from conan.tools.cmake import CMakeToolchain, cmake_layout
class ConanFmt(ConanFile):
name = "fmt"
version = "9.0.0"
settings = "os", "compiler", "cppstd", "build_type", "arch"
def source(self):
git = tools.Git(folder="fmt")
git.clone("https://github.com/fmtlib/fmt.git", "9.0.0", shallow=True)
def layout(self):
cmake_layout(self)
def generate(self):
tc = CMakeToolchain(self)
tc.generate()
def build(self):
cmake = CMake(self)
cmake.definitions["FMT_DOC"] = False
cmake.definitions["FMT_TEST"] = False
cmake.definitions["FMT_INSTALL"] = True
cmake.definitions["CMAKE_EXPORT_COMPILE_COMMANDS"] = True
cmake.configure(source_folder="fmt")
cmake.build()
def package(self):
cmake = CMake(self)
cmake.install()
def package_info(self):
self.cpp_info.libs = ["fmt"]
When I try to go ahead and package this up, like say:
$ conan create . demo/testing -s build_type=Release
This does actually go ahead and clone the {fmt} repo, and correctly build and package up the contents of the library.
Except that:
there is no _GLIBCXX_USE_CXX11_ABI definition set (despite what this page) says.
{fmt} was compiled with C++11, not C++20
... but it was built with Release (-O3 -DNDEBUG)
Conan does appear to provide variables like CONAN_CMAKE_CXX_STANDARD=20, CONAN_STD_CXX_FLAG=-std=c++2a, and CONAN_LIBCXX=libstdc++. But {fmt}'s build doesn't actually use those, so that isn't very useful.
How do I actually get conan to propagate through these fields properly? Is that something that must be explicitly specified in the conanfile.py (even if it's in the profile)?