The quote isn't exactly clear. By "immune", does he mean it in the same sense as the actual definition - that Semirhage simply wasn't affected by the Crystal Throne? Or does he mean it in the more commonly used sense, that Semirhage was able to sidestep its effects?

Also, the actual 'effects' of the Crystal Throne are pretty unclear. Is it so strong as to be compared with compulsion? Or is it simply a trick that is used to impress those who see it, like Mask of Mirrors, only that it inspires awe in its recipients?

Without really know that much about what's going on, I'd say that Semirhage simply wasn't affected, based purely on the fact that 'immune' has certain connotations, which imply a sense of lack of control - that this wasn't something Semirhage planned for and countered. Rather, she simply didn't have to deal with the problem at all - there was no problem for her.

Based on this subjective and biased inference based a single word choice, I'd go so far as to theorize that the 'effects' of the Crystal throne are no more than a trick of the mind, which can be ignored by anyone with a strong enough force of will. I suspect that this is related to the whole deal of Cadsuane's 'breaking Semirhage'. It certainly appears to me that in RJ's mind, he had worked Semirhage up to be an almost unstoppable force of nature, with incredible force of will. A major plot element of Semirhage's breaking was how strong her will is - Cadsuane thinks and treats her like she would deal with herself. I think that this force of will is what made Semirhage 'immune' to the Crystal Throne.