Important fundamentals are omitted but some oddly specific subjects are cherry-picked and given an unusual amount of attention like running nmap. Running nmap (a portscanner) can get you fired from your job and there's precedent of this.
The text formatting is inconsistent. Reader mode makes it better, but a simple markdown file would have clearer.
Explaining what a service is before explaining what a task is, is weird.
APT is mentioned, but APT is specific to Debian-based distros. Debian is not mentioned a single time in the article. Other distros use other package management systems.
You should use nftables instead of iptables. iptables is deprecated.
This article just reinforces that "learning Linux" is simply learning some few random commands.