What does known-good mean?

Known-good meaning a tested and working configuration approved by the developers/maintainers.

What if a security vulnerability is found in one of the dependencies. With an old-style distribution there is a security team that monitors security reports and they will provide a fixed package.

Flatpak is just another model of distribution. There isn't really anything that needs to change here. The bugs are fixed upstream and they get pushed via the method of distribution, which is Flathub in this case.

The security team in a given distribution is charged with getting upstream fixes backported and shipped. There's no need for this role because they're just shipped directly in most cases.

With flatpaks it's not clear to me if those developers will monitor each dependency for security vulnerabilities and how they will handle that.

The developers are usually the ones doing the fixes in the first place.

Will users even be informed about a security issue, will a fix be backported or will it only be available in the latest version?

Well, fixes don't normally need to be backported because flatpaks are usually fresh. They're just built normally in most cases.

For notifications, you'd have to follow the relevant projects directly.