Many Android devices allow multiple user profiles (not to be confused with Google accounts). It's a similar function to what computers have, where you can have the admin account and the child one.
From the second link you posted:
The steps below will only work if your child's device allows multiple
From your comment on this post it's apparent that you're using an Asus Zenpad 3s. That device doesn't seem to have the multi user functionality. (You can confirm this info by simply going settings and looking for "users"). Therefore, you will not be able to add more accounts.
So to explain the difference between the two links you posted, the first one seems to be talking about adding a second Google account to the same device (from settings → accounts → Google,) while the second link is discussing how to get past the limitation of one account per device, by adding a second user, which in turn make it as if you're using a different device, with a lock screen preventing the child from switching between users.
Does this mean two children share one device under Family Link? Or
that there can be one Family Link account and one other account?
With a device that supports multiple users, you can add either a child, or a regular Google account. If you're adding a regular account, make sure to have a lockscreen preventing your child from switching between users.
What are the limitations / implications of a second account?
Had that been possible, your child would be able to open Google play store, switch to the other account, and download any app they want.
Therefore, they don't allow more accounts unless you add a new user to the device.