While I have enormous respect for Elon Musk, I would hope that Boeing engineers have just as thorough an understanding of the workings and safety implications of Lithium-ion batteries. Though Tesla's engineering is admittedly much more focused around battery technology, Boeing has decades of experience with aircraft electrical systems.
no doubt Boeing know that even if the batteries smoke and fail they wont be a threat to safety, but Musk has a better handle on marketing. If there is smoke on a plane or car people expect the worst, regardless of if it is actually a safety issue.
I've seen a Saab in which the ashtray started to smoke and then before we could pull out (on the highway) the dashboard had flames and started to melt.
And I've seen a very nice and shiny Porsche 911 Carrera model 964 whose alternator belt did broke and then started "burning" on the engine. We smelled it and then we could see smoke. By the time we opened the engine trunk (with our fire extinguisher in hand) it was on fire. Funnily enough the repair were under warranty and there wasn't much to change (the trunk needed to be repaint and one or two pieces changed).
When there is unexpected smoke on a plane it is a safety issue.
Seen that a Boeing supplier's factory went up in flames due to a battery that took fire, I'm not exactly sure there's "no doubt Boeing know that even if the batteries smoke and fail they won't be a threat to safety". Same for the 16 hybrid sport cars that burst into flames on that parking lot due to batteries shorting.
I think that: "There's no smoke without fire" may be the sentence you're after ; )