IIRC they aren't based on Flatpak, they existed even before Flatpak gained any traction. I actually do think Steam changed a few things in pressure-vessel to be more similar to Flatpak, but they mostly evolved independently.
The main reason I think they are pouring money into Flatpak is because that is the main way to install applications on SteamOS (that aren't games shipped via Steam).
The main reason I think they are pouring money into Flatpak is because that is the main way to install applications on SteamOS (that aren't games shipped via Steam).