Clever experimental set-up (tl;dr they detect whether novel content first sent between non-China accounts increases the probability of it being censored in realtime when being sent to China accounts -- and it does -- implying a censor (or perhaps a non-realtime automated system) looked at it).

I'm wondering though if this is intentional or just "incompetence/laziness". WeChat itself seems self-interested in not bothering with content transmitted outside China, but perhaps it is just such a small amount of their user-base, that they never bothered setting up a "firewall".