It's compiled code. Seems MyWatch was updated within the last 60 days and the developer talks about fixing things for the 5x in the app store description. Do you know he abandoned it? Maybe he's just busy with other stuff...
Only the developer has the source code, and only the developer has the unique key need to update the app in the store. Even if someone had the source, all they could do would be is use it to upload a different app to the store. The unique developer key is actually there for security reasons, so someone can't take over another developer's app. It's got to have the original developers signing key, which is something not even Garmin has.
If the app has really been abandoned, the only real option is to move on to a different app.