In this case, the title is exact in a subtle way: Pando is the largest known organism by mass, but there are only estimates on the mass of the fungal colony.

Sure, though it isn't like we've weighed Pando either - I would say we just have better estimates of its mass (and of course "largest" is ambiguous as to whether it means area or mass here).

Right, but both the title and the body of the article use the unambiguous "most massive" not "largest."

Depends on how you measure it. Volume, width, wet weight vs dry weight, energy flows, number of living cells, etc.

Fungus covers a large area, but each strand is rather tiny.

Another giant Fungus exists in Michigan: https://www.atlasobscura.com/places/humongous-fungus