Garmin Connect app compatible with Google's Heath Connect app?

You can't, what you have read is people using third party apps like Health Sync to manually pull the data off and push it to Health Connect.

At this point in the game it is very clear Garmin is either massively incompetent with Android development and outsources their app to another firm or are deliberately choosing not to implement Health Connect and likely never will unless Google strong arms developers into using it. I do Android development and have tried out the Health Connect API and I can say that it is very trivial to implement and is well documented compared to their other API's, the hardest part is compliance with Play Store policies and declaring how you are handling the data (which I feel may be a factor on why Garmin doesn't like the idea). I'm willing to guess their motive for not doing it is likely due to some secret exclusivity agreement in one of their partnerships with MyFitnessPal or Strava, or they are trying to maintain their walled garden to prevent people from moving data off their cloud easily.