Well, no because you stated specifically in your post that the R300 series cards gained them 10% market share when the very chart you linked to shows that no, they hardly made any gains.
Have you forgotten already all the complaints about the Polaris pricing? Remember they showed the performance of the RX 480 and people were guessing, based on that performance and the power consumption, that the price for the 8GB version would be under $200 and the 4GB Version would be cheaper again. But, the 4GB version turned out to be $199, the 8GB version was $239. And as far as I can remember those prices were a limited to the first batch of cards and they went up shortly after that once the 4GB versions were sold out.
Do you know the real reason that AMD gained market share for that quarter? It was combination of two things. The first was that AMD had been so long with rebrands that new cards were hotly anticipated and that they had a small window where they had the market to themselves before the 1060 cards were released. You can see on your graph that after the 1060 cards were released their market share slowly started declining again.