So then Mercury waving a big flag about how accounts are FDIC-insured is more about them being made whole if one of the banks they contract with goes under, not about ensuring that their customers are ultimately made whole?
The big risk is that Mercury goes out of business. This leaves the bank with money in Mercury accounts but no way to get back to customers. The funds may be considered to be the companies in bankruptcy, when means that customers would have to wait and may not get back all their money.
The thing you need to Google is "Reg E", which the partner bank (and through them, the fintech program) need to follow. This covers things like losses through card fraud.