In a view how to replace the date of creation of the comment by the date of the flag?
Views filters have nothing to do with tokens. What you need to do is build the necessary query for this, which is something that tutorial ignores completely as it is a pretty simple condition.

A bit of searching pointed me to \Drupal\taxonomy\Plugin\views\filter\TaxonomyIndexTidDepth::query as an example that builds a complex subquery condition in a filter.

I don't know your level of expertise with PHP/Drupal/SQL, so I would recommend you start by building the SQL that is needed for this by hand. In the views settings, enable to show the SQL query on the preview. Then copy that query, remove the {/} execute it by hand and try to adjust it to contain the filter you need. You probably want to add a relationship to flaggging already. 

Once you have that, implement that in PHP in the query method of your filter.