Nope, it's a fungus. To be specific, it's the honey fungus (armillaria genes) that's been recorded to span 2.4 miles... or 12,674 feet. So yeah, there's no comparison between the blue whale and this fungus.
Kinda makes me think of the internet. We are all linked via a massive web of wires. Our thoughts are being shared over vast distances, like one massive worldwide brain.
We are all connected folks. We are all one organism. We, humans, are the true biggest organism on this planet imo.