How do large airliners de-ice in flight?
Putting this as an answer rather than a comment because of its length.

In response to @romkyns comment, perhaps the 787 pilot knew of the heating elements within the wing but was simply telling how ice was normally handled. I really can't imagine him getting through his type rating oral without knowing that.

Had you asked me the same question back when I was flying 747-100 and -200 aircraft, I would have replied thus:

Icing isn't typically a problem because you're usually not in it long enough at a slow enough speed that the ram air temperature rise doesn't get the temperature above freezing.

If you are getting ice on climb out at your normal climb speed, it was usual to ignore it knowing that the icing would soon stop as you gained altitude, and then what icing you had would soon sublimate. If the icing was severe enough to warrant your attention, the usual remedy was to just speed up to get more ram air temperature rise, but you would, of course, be sacrificing some of your climb rate. The U.S. has a 250kt speed limit below 10,000 feet, but you don't have to (or at least didn't have to back then) observe that if there was an operational need to go faster, and generating enough heat to take care of icing was considered a good reason. You normally wouldn't have to think about that though because a 747 climbing out with flaps up has a minimum safe airspeed in excess of 250 anyway.

One indication that the icing was serious was that ice buildup on the windshield wipers would set them to vibrating and giving off a loud buzzing sound. If you let them do that too long, they could depart the aircraft. As I understood it, the time for that to happen was cumulative over the past occurrences as well as the current occurrence, as it was a matter of fatigue.

If you were icing on the approach and didn't want to or couldn't speed up, you could heat the wing leading edge with pneumatic air as described in previous answers. However, it was the leading edge slats that were heated, and when you selected the first notch of flaps, that deployed the leading edge slats, and the heated pneumatic air escaped with little or no effect.

Only once in 10 years of 747 flying did I use the leading edge heating. We were in a holding pattern at low altitude waiting for our turn for an approach into Brussels.