Workflowy encrypts data in transit and at rest, but it does not have end-to-end encryption. This is the same level of encryption used by many popular services, like Slack, Notion and Airtable.
What is end-to-end encryption? (Wikipedia article)
End-to-end encryption is like sending a letter to someone with a secret code. If you send me a letter with a secret code, you want me to be able to read it, but no-one else. So you could have your friend deliver it to me, and know that even if they open it, they won't be able to read it. Once I get the letter, on the other hand, I'll be able to read it, because I also have the secret code.
Workflowy does not have end-to-end encryption
When you type something into Workflowy, it is encrypted before it is sent to our servers, and then it is encrypted again when it is stored in the database. This means that during transit and storage, everything is secret, but Workflowy's servers can still decrypt and read the content.