Short answer: No, that's not the case.
I think what you've heard was a little mangled in the telling. There are indeed only three large airline Global Distribution Systems (GDS), which are the systems where both tariffs (prices) and reservations are created, stored and accessed. These are Travelport (Galileo/Apollo/Worldspan), Amadeus and Sabre, the last of which you mentioned. Even these aren't quite universal though, as many low-cost airlines like Ryanair and Air Asia don't participate.
However, flight search is the next layer on top of this, which attempts to find the best flights and fares in these GDSes. This is a notoriously complicated problem that continues to elude a single winner, and since anybody can purchase a GDS subscription and start doing searches, lots of startups do. So while the GDSes have their homegrown pricing systems, there are also third-party competitors like QPX (one of the most popular packages for doing this), some of the big boys like Expedia roll their own, and then there's a whole slew of startups like Rome2Rio, Adioso and Hipmunk that all attempt to find some edge: Rome2Rio mixes in trains and buses, Adioso lets you do really wide-open searches and combine with LCCs, Hipmunk factors in the "pain" of the itinerary etc.