From the text
OpenCPN on Linux, installed with Flatpak should use "Managed Plugins".
This quote is from bdbcat's intro thread to flatpak.
It is in the text about Managed Plugins. It may be a good idea to repeat it when writing about flatpak.
External plugins will not automatically work
My feeling is that once the managed plugins are really sorted out, and all available, they will be so convenient for the user so no one will ask for external plugins.