AdminContext::isAdminRoute returns false for update.php

Problem/Motivation

When invoking AdminContext::isAdminRoute() in an implementation of hook_preprocess_html(), the function returns false when visiting /update.php. $variables['is_admin'] on the other hand is true. It seems, that isAdminRoute() should also return true in that case.

Edit: Using $variables['is_admin'] in this context is of course completely wrong. I was somehow under the impression it would indicate, that the page was an admin page, although it indicates whether the user is admin or not. However, I still think isAdminRoute() should return true for update.php.