I didn't understand why the existing Patagontitan fossil and this new one weren't considered larger than the largest known Antarctic blue whale at 98 feet.

But it looks like by "largest animal" in this article they mean by weight.

The LONGEST animal is a 150 feet siphonophore, which seems to be some kind of jellyfish looking animal (I'm not sure about where it taxonomically fits on the tree of life though).