Syrian ancient air defences was responding in the direction of the attack, they hit the larges object in that direction. Like I said, Russian and Syrian military assets are not integrated, as such there is nothing that distincts that radar mark as a Russian plane, something that is changing now with the delivery of the s-300. In fact I'm sure Russia actually hopes Israel uses the f-35 more now, so they can gain information about it.
And LOL at all the media reporting it was Syria that shot it down.