Wow, my experience quite the opposite and I've had one or two. In my experience, there's no way DTs locally would try to patch a puncture in the area shown to avoid a certs new tire. 'In my observation', certainly outside repairable area. I'll say this, if they came close to attempting that stunt with me, this is one long time DT customer that wouldn't leave the store without new replacement tire either installed or ordered. Perhaps it's a Cali AT thing.
As for topic DT certs, I have purchased them once or twice over the years but no longer do. I've had such solid results with DT giving me a very fair pro-rated price on a new tire for one with a puncture in a non-repairable area, certs don't make sense 'for me'. I've even had them do it on OE tires a few times, but I'll add that's a YMMV, and in one case some polite negotiation went with it. But, certs can make sense for someone wanting the peace of mind of knowing a new tire going back on with no (or few) questions asked.
As an aside, when it comes to DT tire prorated price towards new tires, as long as tires maintained with recommended rotation and balance interval, several times gotten prorated price towards new set. Last year, helped a family member over the phone to DT store, do that with set of Nankang tires that didn't meet the tread life warranty.