The inline documentation of WP_Query
has this:
Filter results by comment count. Provide an integer to match comment count exactly. Provide an array with integer ‘value’ and ‘compare’ operator (‘=’, ‘!=’, ‘>’, ‘>=’, ‘<‘, ‘<=’ ) to compare against comment_count in a specific way.
So there should only be one array, not 2 like with a meta query:
'comment_count' => array(
'value' => 10,
'compare' => '>=',
),