Honestly I'm surprised they had an answer for you that quick. The two times I've had to contact them they said they needed to "research" the issue and it ended up being about a week before they fixed it.
That said, their answer still doesn't make sense. The high dust is a known issue with the original pad, how much life is left in it doesn't have anything to do with the issue the TSIB is actually "fixing" It's not like they are replacing a "failed" part with a new one of the SAME part and can blame the failure on how worn it is. The problem is inherent to even a brand new one of the part. Their cost in either case is one set of TSIB pads installed on your car.
I'd probably call back and try talking to someone else, making that exact point.
Failing that, assuming your car still has a fair bit of warranty left, I'd just buy the (much cheaper) high dust pads, drive those until they are just short of the "minimum" they are claiming for the TSIB, and then do the TSIB.