The didn’t lose 90%.
It appraised at $400,000. They were forced to sell to the only possible buyer, the local government, at $40,000, instead of getting $400,000 for a proper Eminent Domain Taking. That's a decrease of $360,000 out of $400,000. That's 90%. It isn't difficult math.
What's appalling is that the "modern version" of Eminent Domain is bad enough, and hated by most... but here government is still Taking, AND refusing to even compensate for it!!!!
(And saddest of all, the landowners' primary ally was other state governments who easily foresee this being used by DC to take from the states. It seems that virtually nobody at all was concerned about the acceleration of Taking from individuals.