That answer has follow-up comments discussing exactly your question. The answerer is assuming that the Halachah under discussion (not to recite a Berachah on "forbidden" foods) is worded carefully by the Shulhan 'Aruch and relates not only to foods that have an inherent Kashruth problem, but also to foods that a person has forbidden to himself or someone else has made forbidden to him - ie., through a vow. It is not that the food itself is "unkosher"; but it is still forbidden for the person to eat it.
{It is worth noting that the plain reading of the Halachah is that the Shulhan 'Aruch was discussing "unkosher" foods, and was emphasizing their "forbiddenness" as relates to the level of the prohibition - Torah vs. Rabbinic - not the nature of the prohibition. This seems to be @DoubleAA's main objection, but he is free to comment here to correct me.}