Firstly, Ukraine is not currently a member of NATO, for reasons which have been discussed here already.
But Lieven is saying that, even though the offer of NATO membership was made to Ukraine (and Georgia) in 2008, no plans, even the most basic, were ever made about how it would be defended in the event of a Russian invasion.
I'm no expert, but it strikes me that if you don't make a plan about how you might carry out such a defence, it doesn't indicate much in the way of willingness or ability to actually carry it out.
It appears that many of the actions of various western nations have contributed to enflaming the situation over the years, none of which, of course, excuses Putin's actions, culminating in the Russian invasion.
If you haven't already read the whole of that interview, I recommend you do so.