Are you saying ICE cars aren't at risk of incidental fire from other [burning] cars, what with their 10+ gallon gasoline tanks?
They definitely are (source: former firefighter here), but a deck full of Teslas's is more of a problem for the FD in some regards, than a corresponding deck full of ICE cars. The reason being, Teslas are known to be hard to extinguish when they burn, and are apparently prone to rekindles. I don't remember the exact number, but I seem to recall hearing that Tesla recommend flowing water on a burning Tesla for somewhere around 10 hours in order to cool it sufficiently to avoid a rekindle.
An average car fire (depending on certain details) can be taken care of in somewhere around an hour, from start to finish. So a bunch of Teslas on fire would require more resources committed for more time, which would possibly impinge on the ability of the FD to handle other calls.
Edit: here's the Tesla Emergency Response Guide[1].
According to the "Firefighting" section:
Battery fires can take up to 24 hours to extinguish. Consider allowing the battery to burn while protecting exposures.
Gasoline and diesel (which is harder to ignite) need a source of ignition to start burning, whereas a lithium ion battery can supply its own. The latter being a solid also means heat can build up deep inside a damaged battery pack enough to cause reignition.
Battery fires are notoriously difficult to extinguish.