What are the systems where does this original tar still exist as a
default utility?

If there is no tarfile module on this systems and "tar" version is so
old then you need a more modern system to wrap your releases or more
modern "tar". The ratio of such systems to windows systems without tar
at all is less than 1%, so it doesn't worth the hassle.

But the true reason why I won't revert the part where command line "tar"
is used, to a double call scheme, is that when you do not pipe "tar"
with compression program and use them separately with relative path -
you will get the same weird header in your tar.gz  That's why it is not
uncommon on windows to meet tar.gz python packages with weird paths
inside. Luckily, windows users are infrequently install something from
sources and if they do - the .zip option is often preferred.