It kind of implies that the left join process works by first taking the left table and then filling in the right table with a match if one exists, and otherwise

It kind of implies that the left join process works by first taking the left table and then filling in the right table with a match if one exists, and otherwise null. That model obviously doesn't work because if there's more than one match as the matching left row is duplicated for each match. However I don't understand their point of this being a problem when you don't have a "identity equality join condition", since this can also occur for equality joins as long as you're not joining on a unique key.