Yes, that is mostly correct. You'll need to update the DNS record anytime your public IP changes. HOWEVER, there is an alternative that requires only requires you to update the IP every 2-3 months instead, and this alternative happens to be more secure, so I would recommend it.
My alternative suggestion is to only have your DNS record point to your public IP while you're requesting the certificate from Letsencrypt (running certbot). Once you have obtained the certificate (and configured pihole to use it, as described in the first post in the other thread), I would suggest changing the DNS record to point to the pihole's local IP (192.168.1.50 for example) instead of your public IP. This way, the pihole has a valid/trusted certificate, BUT it is only accessible on your local LAN, and not accessible from the internet, which makes more sense security-wise.
Then you'll just need to update the DNS to your public IP every 2-3 months, essentially whenever you need to renew the Letsencrypt cert (by re-running certbot). Then of course once you've renewed the cert, you can change the DNS back to the private IP of the pihole.
If you want to keep your pihole accessible from the internet for some reason (and therefore keep using the public IP in the DNS record), you can check out some dynamic DNS providers, some of which can automatically detect that your public IP changed and automatically update the DNS record with your new IP. No-IP Free - Dynamic DNS - Create a Free DDNS Account Now - No-IP is one such provider. Again though, I don't recommend permanently setting the DNS to your public IP for security reasons, unless you have a good reason for doing so.