We just set that "Not You?" message using standard drupal message functions (in D7 and D8). A theme will typically display such messages in a separate region above the content region. What you can try is change the Region settings -> i.e. change it from in your case Help -> Secondary.
D8 example - using Olivero Theme: works well:
In Drupal 8 Webform-CiviCRM, this should be easier. Create an element of type "Calculated Twig" and give it the same key name as the "Groups" field. Now you can use Twig calculations based on the rest of the form's values to fill in the Groups field as you like.