Once you found the hidden single 9 in r3, c3, that meant that the 5 in r1 could only be in box (region) 1. That means that the only place for a 5 in column 4 was row 8. That, in turn, places the 6 in r6, c8, because the 6 in row 8 could only go in box 8.
Once you found the hidden single 9 in r3, c3, that meant that the 5 in r1 could only be in box (region) 1. That means that the only place for a 5 in column 4 was row 8. That, in turn, places the 6 in r6, c8, because the 6 in row 8 could only go in box 8.
That was there for almost eight minutes (see the 345 triple in region 1 at 2:32-10:10)- who else was pointing that out? :)
And I follow your logic all the way through; this teaches me to follow through on triples.