They develop a lot of software themselves. They aren't just throwing together a few established libraries and call it a day like 80% of software development. They also take the hard and correct way every time instead of the fast, easy and bad way. Quote from the article:

The same dynamic played out again when Signal introduced support for animated GIF searches on Android and iOS. Instead of quickly and easily integrating the standard GIF search SDK that most other apps were using, engineers spent considerable time and creativity developing another unique privacy-preserving technique that hides GIF search terms from Signal’s servers, while also hiding who is searching for those terms from the GIF search engine itself. We later expanded those techniques to further obfuscate GIF search information by obscuring the amount of traffic that passes through the proxied connection.

When Meta acquired GIPHY, and many other apps were scrambling to contend with the privacy implications of the deal, Signal employees slept soundly knowing that we had already built this feature correctly several years earlier.