It means they haven't done the math. Heck, even if you covered every square metre of a plane with solar cells you couldn't collect enough power. There's not enough there.
Let's do the math, then. The specifications of Solar Impulse-2 are available as a starting point.
At 269.5 square meters of solar cell coverage, and an average power density of 1.35 kW per square meter, the maximum amount of energy the plane can harvest is about 364 kW. Now, we can use two facts to avoid the ugly world of aeronautical engineering (which I don't know): The aircraft has flown under its own power, supplied by four 17.5-horsepower motors. Those motors therefore supply about 13 kW each, for a total of 52 kW of energy required to fly.
Since 52 kW is far less than the 364 kW the solar cells produce, yes, there is in fact enough power available for collection.
Even if you charged up batteries from ground sources you couldn't carry enough storage and have the plane get off the ground because of the weight. Even with an order of magnitude improvement of power density you couldn't.
As noted, the plane has already flown, carrying its lithium batteries with it.
Weight, energy storage density, and efficiency matters too much for that application for it to be any other way.
Ah, finally some thermodynamics! Currently, the whole plane needs an efficiency of about 15% to simply fly. After some quick research, it seems most solar cell technologies today run at about 20% efficiency, with new technologies pushing 46%. Going from the 20% point, that means the motors need to be only 75% efficient. A bit more research shows that they're actually reasonably assumed to be around 85% efficient. The plane will fly in bright sunlight just fine under solar power alone.
Lithium polymer batteries have efficiencies of around 80% to 90%, so going up to a solar cell with 25% efficiency would allow the plane to either charge or fly, but not both. Double the efficiency and you double the capability, so having solar cells that are 50% efficient would allow both charging and flying under ideal conditions. We're getting pretty close to that.
Throw in some assumptions about duty cycles, allowing the plane to be on the ground for a bit (doubling its charging rate, because it doesn't need to spend energy to fly), and making long trips is feasible in several short hops. Account for an intelligent pilot, using tailwinds and other air currents to reduce the energy needs, and those hops can be made longer.
A Boeing 777 is designed for speed. If you're not in a hurry, solar power might just be a reasonable option very soon.