I just tested the official instructions, and they do work. So I’m not sure what exactly you’d want to change, apart from maybe adding screenshots. But more importantly, following the official instructions is actually easier than the guide you linked, especially if you have multiple address books in your Nextcloud.
With the official docs, you simply enter the base URL of your Nextcloud, and Thunderbird will show you a list of all the address books in your account, letting you choose which ones to add. In the guide you linked, however, you have to copy the direct link to a specific address book (already an extra step), and if you have more than one, you need to repeat the whole process for each one, i.e., add them individually.
Only one calendar is displayed here because it is a new test user account. On my productive instance, all calendars are displayed there, including those that have been shared with me. Then you just have to click on “Subscribe,” and the calendars will be added to Thunderbird.