They did it again too: "The clone is about 1.5 orders of magnitude larger than the largest fungi [770 hectares] and the longest sea animal [120 metres]." It's over 3 orders of magnitude longer than that animal, and closer to 1.4 orders of magnitude larger area than that fungus.
But also, it's not a single connected organism anyway (emphasis mine): "a single polyploid clone spanning more than 180 km in fragmented, near-shore meadows".
I think the Washington DC comparison comes from this line: "We used the total estimated area of P. australis meadows in Shark Bay (200 km2 pre 2010/11 heatwave [43])" compared to a DC area of 176km².
Yes, the 200km² vs. 770 hectares is what I used to get my 10^1.4. It's unrelated to the 180km distance the author puts in the DC comparison sentence though.
A correct comparison here would've been to the distance between DC and Philadelphia, or similar.