This from the article's comments (by "wetzel"):

Because it was apparently very expensive and difficult to use, the argument is that a TANG office would never have had one (Selectric Composer). Unclear.

All it took was that there be one year some time after 1966 when there was some leftover money at the end of the fiscal year. A primary maxim of any government organization is not to have any money leftover because it makes next year's budget smaller. In fact, end of fiscal year splurges are a common joke in the military.

And this was the colonel's secretary, one of the bigwigs in the secretarial staff. (Remember that back in the 60s and 70s, extremely talented and competant women did not have the career choices of today, so she may well have been both an extremely hard worker and a genius. It seems she was the kind of person who would switch out the ball to make the superscript 'th').

So at the end of one of these fiscal years, they had a few thousand dollars to get rid of, and the colonel decided to get his secretary the best typewriter on the market. Hell, she deserved it! And this would have the ancillary benefit of letting them produce documents that appeared typeset if necessary (for letters to the generals!)