Even that isn't really accurate. Wechat is an app ecosystem, it is as much like Google Play as it is like WhatsApp. Also, Facebook and google services remain popular in greater china outside of the mainland.
Lot of people in HK use Wechat (I have lived in HK). I wouldn't say it's so clear cut. LINE is also popular. But definitely Wechat is substantial especially since there's a lot of people from mainland and also if you got friends in Shenzen, you will use Wechat. I have seen many HK locals (born in HK, not immigrants from mainland) using Wechat.
Before someone asks, the approximate equivalent of WeChat in Taiwan is Line. Don't know what it is in HK.