When to use mapping or sequence to define an array in a schema file?
A sequence is an unlimited list of identically structured elements. A mapping is a limited set of keys, each key has a possibly different structure.

Your key has node types, so you're storing a list of node type ids, which means a sequence with an inner type string. Exactly what you have. 

If you'd use a mapping, you could only store book and test, but test could be an integer (or mapping, or sequence, ...) while book is a string.

The note has nothing to do with sequence vs mapping, it just refers to an old way of defining the type in the sequence and would look like this:

onlyone_node_types:
  type: sequence
  label: 'Configured content types to have Only One node'
  sequence:
    -  type: string
       label: 'Content type machine name'


PS: I'd would convert that to a numbered list, which will then be stored like this:

onlyone_node_types:
  - book
  - test