Well... this is the definition, apparently:
"The U.S Federal Bureau of Investigation (FBI) defines terrorism as the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives."
So if the actions taken were politically driven (to intimidate Musk due to his work for the US government), or even simply to discourage people from buying Tesla's, then yes it could be terrorism.
If it's just 'cause they had a bad experience with a Tesla then not so likely.
Still a crime, though, and dangerous: Fire's not predictable and is prone to spread - could have put other properties at risk, along with lives, especially of the firefighters, and as they're busy at the Tesla dealership, they can't then respond to another fire... so there's further consequences. That's why arson is such a serious crime.