Arroyo was at 2'00" (official timing) from the trio when he linked up with Evans shortly after the top of the Mortirolo.
With 12km to go (part of the descent and climb is missing from yt) the group of Arroyo, Vino, Evans, Gadret and Sastre is at 45" (official timing).
This means Arroyo must have made an absolute minimum of 1'15" unless the group continued to make inroads on the leading trio on the first 4km of Aprica, which I don't recall (I recall Arroyo being about 40" back at the base and the others catching him quickly, and them holding fast for a while then dropping away).
The descent of the Mortirolo is also 17,0km as compared to the descent of the Col de la Madeleine being 18,5 (not a major difference admittedly, especially considering the Mortirolo descent was wet).
Basso is a bad descender, for sure, but is a Basso being guided by Nibali really that much worse than Andy (who isn't much good) and Alberto (who is mediocre) working together? The other thing, of course, is that Sánchez was only the 3rd story in that stage, after the break and the GC duo; plus of course a great Sánchez descent was expected in the circumstances. Nobody expected Arroyo to pull that out. Plenty knew he was a good descender, but not THAT good that he could make pretty useful descenders like Evans and Vinokourov look stupid.
Samuel Sánchez is an excellent descender, and there aren't too many in the péloton that can go with him if he really goes full bore. I think he started to on the descent of the Navacerrada in 2009, but unfortunately for him Valverde is one of the few that can go with him (Valverde is a superb descender, especially on technical routes), and after he couldn't distance Valverde he didn't try too hard; Valverde was happy with the GC as it was and the two more or less sat up. Arroyo has shown good descending prowess before (eg leading the winning break down from the Col de la Bonette in 2008), but mostly as a domestique (Caisse's team descending in the Dauphiné and Vuelta in 2009 was at times breathtaking, five, six, even seven men all in a line flowing throw each corner). With your career's defining moment at stake, a once-in-a-lifetime chance you will almost certainly never have again to write your name in the history books... that Mortirolo descent was truly epic.