I'm not sure what's not clear about the message. It says the FSDT installer can't be used to install products for FSX or P3D, and is saying it's not a problem, because their installers are always freely available on the FSDT Downloads page.
Yes, you can use the FSDT Installer to UPDATE products for FSX/P3D, but to INSTALL them, you keep using their installers, as you always did.