It is meant to aggregate data from other fitness apps. They just need to integrate with Google Fit
Except the blog post doesn't allude to anything more than what's already present. I can already input weight and run time and it looks like it tracks heartbeat.
Maybe the phone app has problems loading widgets with the webservice currently being flakey.
If I track a 2 mile 20 minute run with another app, Google Fit (so far) only exposes the data as "Ran 20 minutes" completely eliminating the useful information. And if they have a hidden feature that does track distance as well, why isn't that exposed to me to manually enter?
[1] https://developers.google.com/fit/android/data-types or https://developers.google.com/fit/rest/v1/data-types