result

TASK [create inventory/this_is_defined_in_vars]

inventory/this_is_defined_in_vars

[[{'label2': 'muchacho', 'label': 'hola'}]]

test.yml

---
- name: test lineinfile
  hosts: localhost
  gather_facts: false

  vars:
    this_is_defined_in_vars:
      - label: hola
        label2: muchacho

  tasks:
    - name: create inventory/
      file:
        path: inventory/
        state: directory

    - name: create inventory/{{ thisvarisundefined | default('this_is_defined_in_vars') }}
      lineinfile:
        path: "inventory/{{ thisvarisundefined | default('this_is_defined_in_vars') }}"
        create: true
        line: "[{{ thisvarisundefined | default('this_is_defined_in_vars') }}]"

How come the lineinfile "line" expands "default('this_is_defined_in_vars') ? While "path" and "name" treat default('this_is_defined_in_vars') as strings.