Peter H
I also found the claim inherently implausible.
However my first google hit surprised me.
Toxic fluoride gas emissions from lithium-ion battery fire
https://www.nature.com/articles/s41598-017-09784-z
So it would be nice to believe somebody is evaluating "realistic" fires in confined spaces such as aircraft.
Thanks for the very interesting link. I think the statement about 'rather copious quantities' of toxic gases struck me as a slight exaggeration for the relatively small batteries found in most portable devices. In any case many combustion products are nasty, and avoiding inhalation is always a good idea. The relative concentrations would be very hard to assess (even in a laboratory environment). As always, further investigation is justified.