I have always understood that to mean, and be used to mean, that in the absence of explicit statements confirming the conjecture, we have to rely on appearances. So in this context it would mean that while the Yemeni's have not stated that the reason for their actions is to support Palestine, the evidence and obvious appearances lead us to this conclusion.

My issue with the use of the term is that I believe the Houthis have stated as much, so the author sounds like he's ignoring official statements, but it's possible that no official statements have been made, and in that case "ostensible" would be appropriate here.