The author doesn’t mention it but the @<domain> argument to dig specifies what dns server to query. This is useful for learning but also debugging DNS caching problems.
For example, my Unifi router is bad at DNS. When I create a new subdomain my router will not resolve it until I restart its DNS resolver (dnsmasq). How did I figure this out? By doing dig mysubdomain.example.org @188.8.131.52 and seeing the correct ANSWER (184.108.40.206 is Cloudflare) and then doing dig mysubdomain.example.org @192.168.0.1 and getting nothing (that private IP is my Unifi router/DNS server).
dig mysubdomain.example.org @220.127.116.11
dig mysubdomain.example.org @192.168.0.1