WhatsApp user base is insanely impressive but it isn't even no 1 in the country Ycombinator is based in. FBM is. Then it also isn't no 1 in obvious countries like Korea Japan China Taiwan Vietnam.
While I agree that Whatsapp is the dominant chat app in Asia, it's not number 1 everywhere.
Notably - as you mentioned - Wechat in China, but also Kakao in Korea, Zalo in Vietnam and Line in Japan and Taiwan, where very few people use Whatsapp.