The article has the descriptions
RANGE_HI_KEY: Starting boundary for the step.
RANGE_ROWS: How many rows in the base table fall between this Range_HI_KEY and the next (not including either).
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DISTINCT_RANGE_ROWS: How many distinct values between, but not including this RANGE_HI_KEY and the next.
Actually, the RANGE_HI_KEY is the upper-most key for each step, not a starting boundary. And the range is all the values less than the range key and greater than the previous range key (if it exists, it might be the first range.) See BooksOnLine.
Apart from that, great article.