Artworks can be restored, and often have been, from water damage. Much of the art on the main floor of the cathedral is carved stone, anyway, which wouldn’t be damaged.
My theory, for the little it’s worth (well, make that nothing), is that they suspected there was an accelerant, even something like phosphorous, and didn’t want to spread it. The fire fans out almost immediately as if it is following something, perhaps something spread on the framework during the restoration work or planted at various points. But if it is terrorism, the French government is surely never going to admit it, so this will remain speculation.
Notre Dame worker says fire was ‘deliberately set’:
https://www.citizenfreepress.com/breaking/notre-dame-worker-says-fire-was-deliberately-set/