A question over semicolon

Question

I have always thought that a semicolon should separate two independent sentences in a long combined sentence. But when reading this week's The Economist, the usage of semicolon in the sentence below in bold beats me. It appears that this semicolon interrupts the sentence "and for whom he had no interest in keeping it for an exclusive club". Is it OK in this case?

what I think more appropriate is as follows: He was determined to discern what a national theatre should be; and for whom he had no interest in keeping it for an exclusive club.

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The NT was founded in 1963 under Laurence Olivier. When Sir Nicholas took over the organisation, which comprises three auditoriums of differing sizes, it had a reputation for unadventurous repertoire, pricey tickets and an ageing and conservative audience. Public funding accounted for 40% of its annual income of £37m ($47.8m). He was determined to discern what a national theatre should be and for whom; he had no interest in keeping it for an exclusive club. Among other considerations—balancing old plays and new, serious and irreverent, plays that look out as well as in—he wanted to expand audiences and give everyone a “really good time”. [source:http://www.economist.com/news/books-and-arts/21721622-nicholas-hytners-captivating-memoir-his-years-theatres-artistic-director-how]

Answer 1

Another way to say this sentence, which will illustrate that the semicolon is indeed used properly there, is this:

He was determined to discern what and for whom a national theatre should be; he had no interest in keeping it for an exclusive club.

You can see that each clause is independent and that the latter is an elaboration of the former, which is one of the time-honored uses of the semicolon.