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The maximum iron fraction cited for extra-solar planets is based upon the assumption that planet formation is from a heterogeneous material which is differentiated by gravity, followed by giant impacts that remove some of the planet's mantle. Models of the latter removal are used to determine how much of the remaining planet can possibly be iron.
Analysis of exoplanets depend upon simulations and analytical models, trying to match very limited observations with theory. While the apparent density of a planet is often compared with theoretical homogeneous planets (e.g., all water, or all iron), the natural further step is to match the observations of a planet with some accepted means of planet formation.