If the rest of the code looks anywhere near similar to the examples he shows he’s right, this is terrible code and it should never have been allowed in a serious project in this day and age.
It seems like this day and age is precisely when this sort of code is normal. We had a brief window of hopeful optimism in the mid to late 90s and even into the early 2000s, but since then it seems like code quality in general has declined tremendously.
It's not even really a replacement for NetworkManager; it's designed to be used to provide basic networking within containers, for systemd-nspawn. It can be used on host systems, but it's deliberately fairly lightweight and designed for fairly simple configurations in containers, rather than as a full fledged replacement for other networking management daemons.