Gold is the median. Platinum is the high descending curve already.
80% of the playerbase is silver to plat. Bronze is really bad, Diamond is really good.
We had no official numbers since the one I linked above (and you put in your post). Third party sites always have skewed data because they require voluntaries donating their data to the data pool.
If you are using third party data, you should compare the evolution of the numbers with their own data from the past. It’s still not a precise data (especially compared to Blizzard’s data), but you are comparing apples to apples.