All this seems less clear to me than you present here. So far, I've heard left libs bewailing as you say, but others cheering that fellow on for undermining NATO (alas, I haven't paid much attention to who said what).
At any rate, my first impulse is to say that the current situation is a poor example of what withdrawalism might look like - maybe if some one with a more reliable grasp on just everyday reality were doing it...
For instance, I think a withdrawalist could see a vital value in maintaining NATO to keep Russia in check.
Certainly I would be only barely marginally more optimistic if Bernie were doing his version. A continuation of the unevenness, fits and starts of the O years provides a more plausible map to withdrawalism. Enough of everything to make everyone unhappy.
Then if occurred to me that one could take a similar approach to strong interventionism - that GWB was a poor proponent of it, and it is unfair to judge that approach based on his execution.
In the end I see that fellow with no strong commitment to any ism other than that-fellow-ism, which may have tactical similarities to a wide range of isms, but is a poor fit for all of them.