TYPO3 Core

Your first code snippet from the TYPO3 documentation is valid only for TYPO3 v12, which ships CKEditor 5.

In TYPO3 v11 and below, CKEditor 4 is used. You can find the documentation for this version here.
Tip: In the official TYPO3 docs, you can select the appropriate TYPO3 version in the left column above the search bar ("Release" dropdown).

There are many differences in YAML configuration between these versions, some are described in this changelog entry

Beside your already described solution, other possible notations for TYPO3 v11 are:

editor:
  config:
    toolbar:
      - { name: 'basicstyles', items: [ 'Bold' ] }

editor:
  config:
    toolbar:
      - ['Bold']

You can find these notations in the CKEditor 4 documentation.