I can't give you any official answer (I'm here privately and don't speak for the company), but the terms of use of Threema don't disallow reverse engineering. As long as third party clients don't actively undermine the business model or trademarks, this shouldn't be a problem. In doubt, just send an e-mail to info@.

After all, https://openmittsu.de/ is a thing. (Note that the developer of OpenMittsu does not offer the option of generating an identity in the software itself, instead you generate an identity on your phone and then transfer it to OpenMittsu through an ID backup.)

(Edit: Almost forgot: The "Threema Web protocol" has also been officially documented: https://threema-ch.github.io/app-remote-protocol/ I would love to see alternative clients that implement it, to allow using your Threema app from other platforms.)