Honey is, typically, about 80% sugars... Entering the Wheat DME and 6 pounds of honey (a 1 gallon bucket o honey is typically 12 pounds, not sure where the person got 11 pounds for a gallon of honey) into Beer Smith gives an OG estimate of 1.103. With an estimated FG of 1.026, so the alcohol potential is about 10%... Of course, without knowing which (specific brand and strain) yeast was pitched in, it's hard to know what it's tolerance was.
Mead makers will typically add nutrients since there is very little in honey for yeast to munch on (beyond the sugar)... Yeast needs other nutrients besides sugar in order to do it's job. Typically elements such as DAP are used (about 1g-2g/gallon) at multiple times during the fermentation in order to keep the yeast going (until the 1/3 break happens, then it's left alone). Mead is also (most often) aerated during the fermentation before hitting the 1/3 break.
Without knowing the actual hydrometer readings, OG, FG, and step SG, it's impossible to say what really happened. It could be that the yeast simply never really took off due to the higher gravity environment (could have been far beyond it's tolerance level).
I've used up to 5.5 pounds of honey in a brew before, with good results. Of course, I also did it in steps, so that I didn't shock the sheit out of the yeast. I didn't boil the honey either, which can radically change it's characteristics, flavor, and properties (not in good ways either).
I am planning on making an all grain braggot in the future... With 9 pound of grain and 10 pounds of honey (closer to the ratio of a braggot, but not the traditional 1/3 malt, 2/3 honey ratio). Mine will be with MO (normal and crystal) and 1 ounce (total) of EKG (.5 for 60 minutes, .5 for 5 minutes)... I WILL be including nutrients in the last leg of the boil (according to Wyeast directions) and planning on adding more during the first stage of the fermentation. I'm also planning on using a champagne yeast to finish the fermentation. Starting with Wyeast 1728, then using Lalvin EC-1118 once the 1728 has tuckered out. I expect to hit almost 12% ABV with my braggot. Of course, the recipe could change between now and when I make it. Such as going for a full gallon/12 pounds of honey.
For the OP, next time, instead of just pitching in more yeast, make a starter for the yeast. Make a reasonable gravity starter, of sufficient size, so that the yeast will survive (and not get shocked stupid) being pitched into the high gravity concoction. I would also seriously suggest entering a recipe into brew software before going at it. Especially if you're planning something like this again. You also don't need to add ALL the fermentable ingredients into the boil pot at the same time. You could have added the honey after the first week of fermentation, or some of it then, with more being added later. Chances are, the yeast would have been able to handle the additions that way, giving you a higher ABV result. Of course, it could also have been jet fuel, requiring months of aging in order to mellow out enough to drink.
My brew, with the 5+ pounds of honey was started on 12/11/10... It's almost ready for bottles. For the past ~2 weeks, it's been aging with some oak chips, to help smooth things out a bit more.