While I see it as a competitor, I can't see their target markets being the same. Signal, I feel, is targeting average users. Threema's sign up process and verification makes me feel it's for the power user.
I used Threema with my extended family for a while (so not power users). Sign up and (optional!) verification was not a problem. Group management was and ultimately made everybody switch to WhatsApp. Things like "group creator left group, now we must create a new group..." and "identity-only backup means you lose group admin rights".