I haven't read anywhere that Google guarantees to index your site as soon as you submit your sitemap. Submitting a sitemap makes it easier for the Googlebot to index your site.
100% Guarantee? No, although there's a 99.9% chance that all of your pages will be crawled by Googlebots through your sitemap.
I have submitted a sitemap on Google Webmastertools but not all of my pages were indexed when I checked it. I think it'll take time to be indexed by Google.
In theory, all pages submitted within won't be given a guarantee of indexation. In some cases, Google won't index a page when it has already indexed a highly similar version of it before. They try to avoid indexing duplicate pages on their end as well although it is quite evident that they aren't perfect on this yet.
Of course there's no guarantee but from my experience if you submit your entire index to Google they index almost all of it. Make sure your sitemap is constantly updated. WordPress is great for indexing websites.
In my opinion I don't think anything is guaranteed when it comes to SEO. I use the XML Google Sitempas WordPress plugin for the sitemaps on my webpages and it seems to work very good.