Yes exactly, the 3.6 policy specifically mentions 2.6. I can't remember why I originally opened this post asking about backwards compatibility, but I imagine that it was documented somewhere that the new extensions weren't compatible with the old product, but I can confirm that up to recently we were using the 2.6 product with the new extensions and it was fine.
The incompatibility statement makes no sense, because if the new extensions were not compatible with the old product how would you upgrade existing clients - as soon as you checked in the updated extension all of your existing clients all of a sudden wouldn't be managed? If this were the case there would have been a completely new extension set for 3.6, allowing both the 3.6 and 2.6 extensions to be checked in at the same time, which is not the case. If you look at the MOVE guide https://kc.mcafee.com/resources/sites/MCAFEE/content/live/PRODUCT_DOCUMENTATION/26000/PD26120/en_US/... chapter 3 they talk about the process to upgrade MOVE 2.6 clients to 3.6: 1) Extensions 2) Offload servers 3) Clients. I guess McAfee is saying ideally these steps should all be done in quick succession but in the real world it's not always possible. Good luck!