To me this hypothesis seems plausible for the following reasons:
First, the USA and Israel are still reeling in humiliation and impotent rage over their defeat in Syria: Assad is still in power, Daesh is more or less defeated, the Russians were successful not only their military operations against Daesh but also in their campaign to bring as many “good terrorists” to the negotiating table as possible. With the completion of a successful conference on Syria in Russia and the general agreement of all parties to begin working on a new constitution, there was a real danger of peace breaking out, something the AngloZionist are absolutely determined to oppose (check out this apparently hacked document which, if genuine, clearly states the US policy not to allow the Russian to get anything done).
Second, both Trump and Netanyahu have promised to bring in lots of “victories” to prove how manly and strong they are (as compared to the sissies which preceded them). Starting an overt war against Russian would definitely be a “proof of manhood”, but a much too dangerous one. Killing Russians “on the margins”, so to speak, either with plausible deniability or, alternatively, killing Russians private contractors is much safer and thus far more tempting option.
Third, there are presidential elections coming up in Russia and the US Americans are still desperately holding on to their sophomoric notion that if they create trouble for Putin (sanctions or body bags from Syria) they can somehow negatively impact his popularity in Russia (in reality they achieve the opposite effect, but they are too dull and ignorant to realize that).
Last but not least, since the AngloZionist have long lost the ability to actually getting anything done, their logical fall-back position is not let anybody else succeed either. This is the main purpose of the entire US deployment in northern Syria: to create trouble for Turkey, Iran, Syria and, of course, Russia.