The way Sierra gets that weight on these tents is to use superlight (won't take much abuse) matierals--20 denier and 40 denier fabrics. Now, these light fabrics that are silicone coated are quite strong, but typically tent manufacturers use a polyurethane coating or silicone/poly mix and these can drastically reduce the strength of the fabric where the silicone (stretches with the fabric) ends up being much stronger in tests. Judging by the 3000mm watertight floor ratings, I'm guessing that Sierra is not using silicone. It's atypical to get that high of a rating if using silicone, but they don't specify. So, I'm always a bit leery to go to light on these materials. There are some new coatings, but I'm not sure what they are doing to fabric strength. The Mojo also uses a partial fly it appears--this is a long time Sierra tactic that does lighten the tent, but not without consequences. This means that part of the tent body itself will be made with a 'waterproof' material as it is directly exposed to the elements, which means that part of the tent wall will be more likely to develop condensation on the inside, in your living space, because if it's watertight material, watervapor cannot exit the tent wall there. The Mojo also has just one door, it appears, and an end door at that. Personally over the years, i've become a big fan of the large and easy entry and exit side doors and double vestibules so that each person had their own door and vestibule to store their gear. Just makes life in the tent more comfortable and enjoyable.
The Marmot Earlylight (3p version as well) uses 68 and 70 denier materials that will be stronger and likely provide more years of service if taken care of, not that the Mojo wouldn't do the same if treated very well--it just has less of a chance to do so from the get go due to manufacturer fabric choices in an effort to show a reduced weight. There's nearly alwasy a trade off for this. I was looking at kids using the Earlylight and thought that it might be a good idea not to shave weight too much in the fabric department with kids that might not fully grasp the concept of being very careful with the tent.
Also, take a good look at the Tarptent Double Rainbow: Tarptent Ultralight Shelters
At 2lbs. 9 oz. it's hard to beat. The floor is not quite as durable and watertight as I'd like (get a floor saver), but many are very, very pleased with the design. I own the regular 'single' Rainbow and think that for a very comfortable, 3-season, 1 man tent at 2 lbs. 3 oz. it's very, very hard to beat for $245. Henry buys the same materials from the same supplier that I do and I know that the silicone coated fabrics he uses are very strong.