Hi there,
this line:
if ( class_exists( 'Classic_Editor' ) ) {
is where we check the Classic Editor is being loaded.
So you could try removing the condition and just load the styles regardless:
add_filter( 'generate_editor_styles', function( $editor_styles ) {
$editor_styles = array( 'assets/css/admin/editor-style.css' );
return $editor_styles;
}, 20 );
Alternatively you would need to ask the plugin author what conditions we could use instead.