I’ve interacted with them on their forums (https://forum.zettelkasten.de) and with regard to The Archive (The Archive (macOS) • Zettelkasten Method).
They are both very approachable and generally interested in talking about PKM. Christian can be found on his page: About Christian • Zettelkasten Method and Sascha on his: About Sascha • Zettelkasten Method.
Their blog is a very mine of ideas about PKM: Blog Post Archive • Zettelkasten Method, with posts going back to 2013. If you’ve never looked at it, you’ve missed something.
Christian actually made a short post about Tinderbox back in 2018: Zettelkasten Method and Tinderbox (macOS) • Zettelkasten Method. He might conceivably be interested to be brought up to date about recent developments.
Edit: I posted on their forums: Using Tinderbox for a Zettelkasten — Zettelkasten Forum