You can download your data [1] and delete your account [2]. These are two things I always check for before trusting anyone with my data.
[1] https://help.fitbit.com/articles/en_US/Help_article/1133
[2] https://help.fitbit.com/articles/en_US/Help_article/1285
What you can’t do is delete the data that Google is collecting on you without an account. Data your family gives them. Data from your emails that land in gmail inboxes. Data from the umpteen thousand cookies that lead back to Google.
I’d like to see what Google has on me, but without an account I can’t.
but now that could change
It's a good lesson about being willing to share data: even if you trust the company you're sharing with, a company you don't trust may end up buying that company and getting your data after all.
If you're privacy-minded, the only safe option is to treat all companies as untrustworthy.