No, you don't get it at all. It was a mistake only because it weakened HIS position (by dropping a teammate). Contador's GC position was the ONLY one that mattered. This 1-2-3 boloney is a bunch of crap. Contador (and Astana as a whole) needed to be concerned only with winning the Tour, not worrying about where their 2nd and 3rd riders were placed. It was a mistake because it left HIM isolated with the 2 most dangerous riders to his GC standing (because they would work together agaisnt him, not because Frank was actually a threat).
This also why Armstrong failing to properly mark Frank was a mistake as well, and a huge one. It left Contador with TWO strong Saxo riders to deal with instead of one. If Armstrong had marked Frank, Frank would not have been able to bridge because it would have been worse for Saxo to have both Armstrong and Frank in the lead group than both in the chase group, so Frank would NOT have dragged Lance back to the lead group.
Contador's attack opening the gap did not hinder Armstrong from bridging, either. He DID try to attack Wiggins lower on the slopes (after Armstrong let Frank go but before Contador's attack widened the gap), but Wiggins followed the first attack and Armstrong HAD to ease off. It wasn't until near the summit that Armstrong was able to get rid of Wiggins, and even if the gap was still under 2 minutes, there's no way he could have bridged it. He made up less than a minute on the downhill chase.
There's a lot about this stage that you clearly do not understand. It's best that you just concede to Publicus on this. He has the situation nailed dead on.