How can I access webform submission values from webform-confirmation.html.twig
You can access submissions using 'webform_submission' variable. I know the webform-confirmation template file doesn't mention that variable but, if you look closely to the controller you see there additional variable being passed to this template. 



N.B on your form setting make sure to set confirmation type to be "age (redirects to new page and displays the confirmation message)" 

And always the latest one on the array is your last submission.