Not a surgeon, but I agree with B1G. Remember that the first maxim for a doctor is supposed to be "First, do no harm." Drilling holes through two bones is invasive, with the attendant risks of infection and anesthesia (says the guy who's been put under 30+ times). Also, there's no material which could be used which would withstand the 90° bend which Tua's ankle was subjected to. The biggest advantage of the polyester which they use is the "give" which the older screws and plates didn't have and which approximates the natural ligament behavior. That much flexion would just tear out the polyester tie...
Not a physician (nor do I play one on television) and have no inside knowledge of Tua's situation. I do come from a medical family (physicians, nurses,, etc) and I've spent many years in clinical medicine myself, including a lot of ER/OR work.
My thought is that Tua mostly likely has (had) a severe grade one or mild/moderate grade two sprain. Certainly in most cases you would not consider a tightrope procedure for a grade one, and in many cases not for a grade two.
However, even in cases with a bad grade one high ankle sprain, you're looking at 3-4 weeks recovery (RICE - Rest Ice Compression Elevation) before Tua was back to practice. I think the medical team may have discussed with Tua and his family the procedure as a way of speeding up the recovery process, not necessary a "prophylactic" treatment to strengthen the ankle and prevent future injury. My guess, and it only a guess, is they left it to Tua and his family to decide.
I guess it could be similar to a meniscus injury in the knee. Many times players, with tears to the meniscus, will rest it a few weeks, brace it and come back to play the rest of the season, putting off surgery until the end of the year. Problem is one of needing to deal with the pain/swelling/inflammation that comes with use after practice and games. Other times they choose to scope it, clean it up, and get back to play within four weeks or so. Biggest difference is that with arthroscopic procedure you're not leaving the hardware in place like with the tightrope.
I think that it was an option for a "more aggressive" approach to possibly speed up the recovery time. In normal cases, you or I would be advised to the more "conservative" approach and a four to eight week recovery timeline, depending on the severity of our sprain.
But you nor I are the QB on a top ranked team with the prospect of being a top pick in the NFL draft in the spring.