No you are wrong; the message that pops up that says you need to download new add-on manager is while you are in the sim-it never says that MS FS X needs to be off .
That's not the case. You are confusing two messages:
1) There's a first message, which is the Live Update notification, that will be different depending if we published a Minor Update or a Major Update.
If we publish a Major Update (like in this case), the message WILL tell you to EXIT from the sim and download and install the Stand-Alone Addon Manager. If we publish a Minor Update, it will NOT tell you to download the Stand-Alone Addon Manager, and will tell you the update can be installed without exiting from the sim.
Since we published this update as a Major Update, you should have seen this message, which prompted to EXIT from the sim.
2) There's ANOTHER message, that is NOT the Live Update notification, but an additional check that is being made (in case you ignored the Live Update or disabled it) which checks if you have the latest Addon Manager, which will just tell you to download the Stand-Alone Addon Manager, starting the browser to the download page.
In this case, it's true that it doesn't say anything about quitting the sim so, we'll update the web page accordingly, so it will be more clear.
it was because it did not tell you to turn off MS FS X before trying to download the new add-on manager!
I said that because the antivirus is by far the most common case of that problem. Most users know that, in order to install the Stand-Alone Addon Manager, you must exit from the sim, so it never occurred to me that someone would try to do that but yes, it's another possible cause for that message.