Interesting find. I was testing it with inline CSS where custom fonts seem to be allowed but not in external stylesheets.
Anyways, I thought about it. Since using AMP itself means your pages will be loaded via a Google CDN, it’s irrelevant whether Google Fonts are served there since the request is being carried out by Google and not your website in particular. In which case, you can use the following code in your themes functions.php or in a plugin to disable the processing done by the plugin:
add_action('wp', function() {
if (is_admin() || !class_exists('\Sphere\SGF\Plugin') || !function_exists('is_amp_endpoint') || !is_amp_endpoint()) {
return;
}
remove_filter('style_loader_src', [\Sphere\SGF\Plugin::process(), 'process_enqueue']);
add_filter('sgf/process_css_files', '__return_false');
});