Users used bad passwords. Their accounts where accessed using their legitimate, bad, passwords.
Just as an anecdotal counterpoint, I am a 23andMe customer who did receive notification of my account was accessed and personal information obtained.
This was my password at the time: 7Kk5bXjIdfB25
That password was auto-generated for me by the BitWarden app.
So for what it's worth I don't think my password was a 'bad' password.