WordPress Search can work for BILLIONS of Documents?

I was surprised when I read this:

There are open-source scalable search solutions – like Elasticsearch and Solr – that work with WordPress to optimize search indexes. They can deliver near-instant search results across millions, even billions, of source documents.

I really don’t think that I’ll be using WordPress to catalog Millions or Billions of Documents.  I’m guessing that I’ll be using either SharePoint (the free edition) or SQL Server ‘Full Text Search’.  Maybe I’ll try using WordPress for some of these purposes, on a Virtual Machine and then I’ll see how it responds.

I love WordPress.  But I do love SQL Server a LOT more.

How scalable is WordPress?