The Valdez was an oil tanker, carrying oil. This ship is a coal carrier, with just a little oil for it's own engines. It could spill all of it, and while it wouldn't be pretty, it would not came with a couple of orders of magnitude of the Valdez spill. Plus the Valdiz spill was in sheltered waters, this one not.
I make 900 tons of heavy fuel oil out to be 248,000 gallons,
As you say, no Exxon Valdez but still one hell of a gookie mess. That's about 31 over-the-road tankers of oil!