(The content below is just based on my knowledge and understanding. Feel free to correct me if I'm wrong.)
I just read the passage of your link and the Zoom Cryptography Whitepaper. The post-quantum cryptographic algorithm mentioned in your passage (Kyber-768) is an asymmetric algorithm, while Joplin uses a symmetric algorithm (AES) to encrypt the notes. As noted in Wikipedia,
most current symmetric cryptographic algorithms and hash functions are considered to be relatively secure against attacks by quantum computers.
You can also find that Zoom uses AES to encrypt the meeting streams and emails. It's documented in their whitepaper.