Yes, the wings of the glider provide lift that otherwise would not be present. The engines still need to produce more thrust to pull the glider but that's expected, the extra cargo doesn't come free.
That... kind of makes sense. But (the way I* see it) modern large aircraft aren't very overpowered (otherwise they would be less efficient), so their engine power is just a little bit above what's needed to get them into the air with a "usual" runway size in ~ the worst possible conditions ("hot & high"). Which means that they would need ~ double the runway length to take off with a glider in tow? So this works with the small aircraft they're currently testing with (they just have to use a longer runway), but if they want to extend it to larger cargo jets, they would risk exceeding the limitations of existing airport runways?
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* - someone who has an interest in aviation, but no professional background or training therein
