The judge never said MAL is worth $18 million. Trump stated it was worth between 400-600 million. Trump's side had to substantiate that. Trump's side brought in an expert. This expert said it was worth $1.5 billion. Trump's expert was asked how he could substantiate that and he couldn't. Trump's expert brought forth no formal proof, documentation, or analysis of any value. As a result, the judge had to refer to the only "appraisal" that was entered into evidence, which was the property tax record.
I've dealt with property tax disputes throughout country and if you don't bring proof, you don't win. Period.
Fwiw, Forbes recent appraisal said MAL was worth $325M.
I don't think Trump materially intentionally misrepresented MAL value but in 50/50 states, you have to support values with facts and documentation. Trump's legal team and expert did not do this here.