Class not found

Yes. I have require 'vendor/autoload.php'; at the top of the module. I just noticed that these two lines also appear in the module:
use Sendpulse\RestApi\ApiClient;
use Sendpulse\RestApi\Storage\FileStorage;

As I said above the actual php files were installed by composer at
/opt/docker-substantiator2/vendor/sendpulse/rest-api/src/ApiClient.php
/opt/docker-substantiator2/vendor/sendpulse/rest-api/src/Storage/FileStorage.php

Do I need to even have a use statement?

proof that an old dog can learn new tricks