I really disagree with this.
Google has extremely strict safeguards in place to prevent eg. employee Joe from accessing ex-girlfriend Mary's Gmail. Very few people would have full access to individuals' Google accounts. This kind of privacy breach would be very damaging to Google.
Niantic is a tiny startup with around 50 employees. I would expect most developers within the team would have full access to the production database, or would be given access if they had a need for it. It's unlikely that there's any oversight over who can access data - it's just a 'game'.
Where are their backups stored? Are they encrypted? Who has access to the decryption keys? We don't know, but I would bet any amount of money that their systems are vastly less secure than Google's are.
The relationship between Google and Niantic isn't relevant as to whether they are capable of keeping these credentials secure.