No API available.
Seems like this might be possible now: https://support.welltory.com/article/show/98292
Thanks for sharing that! This seems to make a one-off request for data from another app on Android or iOS possible, but doesn’t allow long-term integration to regularly sync data. It’s a bit of a cop-out on their part, I feel, and doesn’t make it any more possible for us to use as an integration unfortunately.