Sage WordPress - Plugin Namespace: Not Found

I originally posted in the Sage forum here.

I'm trying to get a plugin to work but it's not, and I feel like it's a very general Namespace problem, and not something with the actual plugin, so I feel I could find success here.

The plugin (and it's code/repo) is here: https://github.com/ItinerisLtd/acf-gutenblocks

Any ideas how I can solve this error?

Fatal error: Uncaught Error: Class 'Itineris\AcfGutenblocks\Plugin' not found in /app/public/wp-content/plugins/acf-gutenblocks/acf-gutenblocks.php:32
Stack trace:
#0 /app/public/wp-includes/class-wp-hook.php(286): Itineris\AcfGutenblocks\{closure}('')
#1 /app/public/wp-includes/class-wp-hook.php(310): WP_Hook-gt;apply_filters('', Array)
#2 /app/public/wp-includes/plugin.php(465): WP_Hook-gt;do_action(Array)
#3 /app/public/wp-content/plugins/advanced-custom-fields-pro/acf.php(334): do_action('acf/init')
#4 /app/public/wp-includes/class-wp-hook.php(286): ACF-gt;init('')
#5 /app/public/wp-includes/class-wp-hook.php(310): WP_Hook-gt;apply_filters(NULL, Array)
#6 /app/public/wp-includes/plugin.php(465): WP_Hook-gt;do_action(Array)
#7 /app/public/wp-settings.php(525): do_action('init')
#8 /app/public/wp-config.php(76): require_once('/app/public/wp-...')
#9 /app/public/wp-load.php(37): require_once('/app/public/wp-...')
#10 /app/public/wp-blog-header.php(13): require_once('/app/public/wp-...')
#11 /app/public/index.php(17): require('/app in /app/public/wp-content/plugins/acf-gutenblocks/acf-gutenblocks.php on line 32

I put the following code into my setup.php:

add_filter('acf_gutenblocks/blocks', function (array $blocks): array {
    $new_blocks = [
        Testimonial::class,
    ];
    return array_merge($blocks, $new_blocks);
});

If I remove the code above, my application works correctly.

My file structure, where app lives in wp-content/themes/[themename]/app:

app
-- Blocks
-- -- Testimonial
-- -- -- views
-- -- -- -- frontend.php
-- -- -- Testimonial.php
-- setup.php 

In my setup.php I have the following code:

Link to Gist with my current code here (setup.php, Testimonial.php, Frontend.php)

You'll notice also I put protected function registerFields(): array {} into the bottom of Testimonial.php, I hope that's a normal place to put it.

For the setup namespace, I've tried different variations: use Blocks\Testimonial\Testimonial; use app\blocks\testimonial\testimonial; (with lower case filenames also)

Edit The best idea I've got so far is that I've installed the plugin ( composer require itinerisltd/acf-gutenblocks ) to the directory sitename/app/public/wp-content/plugins/ and maybe the files in the Sage theme aren't picking up the namespace because it's in the main WordPress plugins.