Google's been investing in medical data analysis long before buying Fitbit. They got in trouble for DeepMind's involvement with NHS data over in the UK, which started years ago. And then Google absorbed that whole project from DeepMind into Google Cloud proper, after assuring the British government Google would never have access to the data from the project.
tl;dr: Unrelated avenue in a field they've been interested in for a long time.
I suspect the Fitbit acquisition is more to have a product that competes with Apple's and Samsung's smartwatch offerings. At this point it seems like having a watch that integrates with your phone offerings is table stakes, and I don't think Google had that.
I'd expect it's coincidence but in the other direction; Google's Fitbit motion has led to journalists being curious what other moves Google is making in the medical space.
tl;dr: Unrelated avenue in a field they've been interested in for a long time.