That's bizarre. I just looked at the app and they support contacts-only limits for everything (voice, presence, profile photo, forwarded messages) _except_ for regular messages.
You can't reject images from non-contacts either. It's either images from everyone or nobody.
Does your chats list get filled up with these unsolicited messages from randos?
It seems like they should at the very least offer a mechanism to accept a contact request before any messages can be sent.
It's also an interesting example in the vastly different experience that different demographics can have in an app - say, women, or minorities often the recipients of abuse - and how blind an app maker can be to that experience if it's not intentionally studied.