What's the advantage of this over a whole-system provisioning system like ansible?
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From what I can tell, you wouldn't use it instead of Ansible or another automation system, but rather just support for a config file you can plot in to make setting up automation with any automation system easier by allowing you to put it into a file rather than a gigantic Flatpak install command.
Flatpak is unironically dead. It provides next to no security benefits. Single store with no verification of updates. Worse compatibility with other system apps. Its only positive is that it gives developers a single linux platform to target but it even does a bad job at that since the developer experience is subpar compared to snap and app image.
Apart from having a good name its kinda bad at everything.