I believe we must remember the “fire triangle” here when wondering “Why, if trees covered by lava, would they still have their bark?”
To have combustion, one must have 3 elements: A heat source (the lava), fuel (the bark) and oxygen (not present if the trees were covered quickly by the lava; the lack of oxygen is further shown by the presence of the wood itself, for if there was sufficient oxygen to consume the bark, the entire tree would’ve been consumed).
To deny that these trees are surrounded by lava would be to deny basic geological practices that can determine what kind of rock a rock is. Thus, we must conclude they were covered by a “flood of lava”, not a flood of water. The presence of the bark is inconsequential and fully explainable by the logic above.
Most of what you say is correct. I do have one question though—if they were buried by lava as fast as you (and the article) claim, why are they all still standing? That much lava moving that fast would knock most—if not all—of those trees down.