This software is not developed in the open, so only its developers know how it works. It may be insecure in ways that are hard to detect, and it may change without oversight.
tbh assuming automatically open source software is more secure is false, At least one link which mentioned studies said that open source probably does not always “outperform” closed source software in term of security.
The relative security of open source software has been examined repeatedly by researchers since the early 2000s. Open source software contains no more flaws on average than proprietary software. In some cases, it may have fewer vulnerabilities.
Some might argue that having a paid team means better security, So i don’t know if the warning about security is really justified, and it might give people a bad impression about flathub (that it is being dogmatic), at least link to some page providing a more detailed explanation would be better (and might prevent new FOSS users from getting a false sense of security).