ddev pull fails on Wordpress Pantheon site (doesn't pull files)

[rfay]

Describe the bug

If you try to do a ddev pull on a properly configured Wordpress Pantheon site, you get

An existing user-managed wp-config.php file has been detected!
Project ddev settings have been written to:

/Users/rfay/workspace/ddevwptest/wp-config-ddev.php

Please comment out any database connection settings in your wp-config.php and
add the following snippet to your wp-config.php, near the bottom of the file
and before the include of wp-settings.php:

// Include for ddev-managed settings in wp-config-ddev.php.
$ddev_settings = dirname(__FILE__) . '/wp-config-ddev.php';
if (is_readable($ddev_settings) && !defined('DB_USER')) {
  require_once($ddev_settings);

Even if you then add the settings as requested there, it continues to pester (and apparently to return a failure)

To Reproduce

Basically, this will happen with any existing wp-config.php. (You don't need pantheon).

Removing the wp-config.php and ddev config or ddev config pantheon will get you going just fine (ddev will then create its own file)

I used the ddevwptest Pantheon project (available to the drud account and my own) and was easily able to demonstrate this, even though in the end it's not about Pantheon. But it does prevent the project files from being downloaded, because the config process fails.

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Version and configuration information (please complete the following information):

• Host computer OS and Version: [e.g. Windows 10, macOS High Sierra]
• Docker version information (use docker version) and insert it here.
• ddev version information (use ddev version)
• config.yaml contents for the misbehaving project
• Do you have any custom configuration (nginx, php, mysql) in the .ddev folder? If so, have you tried without them?

Additional context
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