They are probably talking about unqiue users in each app. 10b/18 ≈ 555m per app which isn't unreasonable for Tencent.
According to their chinese doc, the number is actually "multiple hundred millions", so it's more likely just a translation issue.
Chinese (and japanese) use 4 digit separated factors, so it's easy to make these kind of mistakes.
eg 100 ten-thousands not 1000 x 1000s. Even after living there awhile, converting big currency numbers is a pain.