Since you would be using Bedrock (implied by use of Trellis), instead of wp-config.php the variables would be stored in .env (at the site root) and if they needed to be manually retrieved you would use env('PLUGIN_API_KEY_ID').
I saw the ACF guide and tried it before posting but it didn’t work. I can see the constants in the .env file. Now I suspect the problem could be the plugins fault. In this case Offload S3. Their install guide says the constants should be defined before loading wp-settings.php, and I see config/application.php loads before it and should have the constants defined by the time the wp-settings load. I will keep looking for clues and will update.