The text underlined in red and the text underlined in orange do contradict each other, or at least do not support each other. This is not surprising, they are from different texts by different authors, who apparently do not agree on this issue.
The text in red said that it seems strange to impose a rule on the parties that they never agreed to, or even implied agreement to. The orange text says that doing this may give a just result, and that higher courts have approved this approach.
Both texts agree that Steyn should not have discussed mutual mistake once he found that there was a condition precedent, because that settled the matter.