I am not sure, but I think the IDs count posts and comments. So I think that means about 4 million submissions and over a million comments. Not all submissions draw comments.
Look at the URLs for a submission (found via its "discuss" link on the post listing page) and for a non-post comment. They are identical in format; only the ID value varies.
For this to work, they must share the same set of ID values.
For this to work, they must share the same set of ID values.