Retired Navy Chief here. I would think “assistance” would also include defend. I believe the loophole is that Ukraine was never voted into NATO. That may be why Obama did not pursue NATO when Putin took Crimea, I don’t know. Perhaps NATO was tired of helping U.S. in wars.
Very respectfully, you would be incorrect on that assumption. I negotiated treaties for a living. The "loophole" here is the absence of a positive obligation to do anything but seek UNSC action. NATO membership would of course provide Article 5 protection beyond the Memorandum.