Yes, this is correct - Wordpress SEO works like this. If you want to disable pages, posts or attachments, or add tags or categories, those options are on the XML sitemaps options page for this plugin.
Here's a writeup on why it's no longer named sitemap.xml:
http://yoast.com/xml-sitemap-in-the-wordpress-seo-plugin/
Probably search for, "Which sitemap should I submit to Google Webmaster Tools" and read that & the next FAQ item.
Personally, I just grab the individual sitemaps and add those to my Google Webmasters account.