It's not believable that the Israelis would risk an expensive new plane only days into testing on missions. The only "source" is a French journalists, and numerous other experts have expressed substantial doubt about his story.
Why? Is Israel paying now?
http://mondoweiss.net/2016/12/israels-free-ride/
Also worth noting that the 2nd link above says Israeli media reported it too. Are there any credible denials which suggest which aircraft was actually used? Everything I can find only says it's "unlikely" but won't go further. Who are the numerous experts? Why would they use aircraft which aren't stealthy while Russia has an ever-increasing presence in Syria?
The other option is the US did it with the plausible deniability that israel would be blamed in order to test the f-35.
I mean, if it were you, wouldnt you want to test the damn thing in an operation after all the spending and PR the plane has received. Even behind closed doors you'd want to be able to say "See! It works"