According to things I discovered on their privacy pages here:
https://www.notion.so/willchatham/Terms-and-Privacy-28ffdd08...
They encrypt your data at rest, have their stuff audited by a third party quarterly, and give the employees some amount of security training.
The thing I am curious about is that to log in, you don't have a password. Rather, then send a temporary login code to your email address. Not sure how I feel about that.