Yes that’s by definition domestic terrorism, Elon is part of government and even if he weren’t so long as the fires and such are intended to raise awareness by fear or loss of profit it would still be by definition domestic terrorism.

For reference according to the FBI since 1982:

The FBI defines terrorism, domestic or international, as the unlawful use of force or violence against persons or property to intimidate or coerce a Government or civilian population in furtherance of political or social objectives.