Everything that was written in this and Ars Technica's article can be explained in a single term: Android compatibility layer.
This comment from xda sums up my thoughts perfectly so I'll just repost it here.
"It is fake - android is there similar to how linux is available in windows through wsl, but using the same kernel (Linux) and with deeper integration through Ark compiler engine and similar stuff. Android support is completely pluggable.
Also who said adb works natively in Harmony? What if they use an own debug protocol (should check that). Android subsystem most probably works in a sandbox with optional bridging.
It is insecure to modularly plug a set of apps from an other os, which has a completely different security model, into host os. You will definitely sandbox it, resulting in adb seeing its own, dedicated filesystem tree, withouth having access to harmony os filesystem except when given perms by user.
Phone harmonyos is still linux based after all, and thus it has chroot and all these cool things like namespaces, cgroups, etc.
Another example is Anbox - it is a containerized Android runtime for Linux. It uses host kernel through the host kernels isolation methods i just mentioned above. And, oh wow, if you adb to the pc with anbox, you will see android directory structure, not host pc one.
To the author: please, goddamn stop ignoring *official* documentation made by Huawei and Linux kernel developers, knowing that stuff will prevent you from making such articles."