It turned out not to be the issue,
but rather a "subapplication" within the main application
was autoloading an earlier version of guzzle.

So this turned out to the result of poor design
and then having 2 composer autoloaders mixing up with eachother.

Running composer update in the subapplication
resolved the issue, as it was autoloading the functions.php
from the older guzzle version:

  - Updating guzzlehttp/guzzle (6.3.3 => 6.5.2): Loading from cache