The redirect method made the wp-config.php file loaded by WordPress used the require() function to bring in out config file, which we maintain at config/wp-config.php. This was causing WP CLI to fail because it actually goes into the WordPress config file and reads it line-by-line. Since our config file contained only a require() statement, it was failing. I've created a symlink from vendor/wordpress/wp-config.php to config/wp-config.php to solve the problem.