In that tutorial they suggest running
certbot certonly --standalone -d foo.example.com
To reissue that certificate to cover the additional domain, you would run
certbot certonly --standalone -d foo.example.com -d xyz.mydomain.com
At that point you would need to repeat the subsequent steps in the tutorial related to copying the certificate into the appropriate place for your web server.
One thing that I don’t really like about this tutorial is that the suggestions there won’t automate the renewal of your certificate. The tutorial does correctly point out that the certificate will expire after 90 days and that it has to be renewed by then, but it seems to suggest doing so manually by running the command again. Certbot has lots of features to help automate this process so that it can perform the renewal automatically, but the tutorial doesn’t mention this or suggest the value of taking advantage of this.
If you’re interested in that issue, I would suggest starting a new thread here like “How can I automate renewals with Tomcat?” and linking to the tutorial you followed again. I’m sure that people will help with advice about automating the renewal process.