I guess they are if you are able to see and write to contacts that are using the playstore version on their android devices.

Update:
You actually need to have SMS support activated in the settings for contacts to show up on the Jolla.
Though It does not seem to differentiate between normal contacts and TextSecure contacts.
I guess its a bug in Alien Dalvik as it is working fine on BlackBerry Android Runtime (Yeah I also own a BlackBerry)

However if you click on a contact that is a TextSecure contact it shows you an indicator in the chatbox that it is a TextSecure contact so the message is not send as SMS