I ran into this issue because apparantly the FSDT installer program is the place to activate (not yet download) and the AddonManager program has not been updated with a warning that the activation function is no longer valid.
This is just your assumption, which is wrong.
The Addon Manager can STILL be used to activate any FSDT product in FSX/P3D so no, no "policy" has changed here. What you are getting is a PROBLEM, because some of the updates which are required hasn't been installed correctly for some reason.
That's why I listed a procedure that should fix your problem.
If you find easier to use the FSDT Installer to activate, sure it is, but you can still use the Addon Manager, nothing changed.