I have many RSS feed podcast which gives only 100 items per page and link to the next 100 items. However in podcast Addict when I scroll down I can only see top 100 episodes not more than that. There is currently no way to see the items of subsequent pages.
Podcast Addict displays the full content of the RSS feed, not less, not more.
In your case it means the RSS feed as been limited to the 1000 last episodes. Other app may server based, that’s why you might be able to retrieve older episodes. By server based I mean that they are not parsing the RSS feed in the app but on server side, while storing past episodes. The problem with this solution is that when the server is killed (like Google reader) apps based on this solution won’t work anymore.