> I wonder how temperature is measured for an object this far away.

From the black body radiation spectrum.

[UPDATE] It turns out that the temperature of TrEs-2b is not directly measured, but extrapolated from other measurements (at least according to Wikipedia - https://en.wikipedia.org/wiki/TrES-2b#Temperature ).

> Instead of the energy being absorbed as heat by the planet, it'd instead be stored in some other form or used for interstellar travel, construction etc, right?

Yes, exactly. So the only possible impact of energy harvesting would be to make the planet cooler than it otherwise would be. How much cooler depends on the efficiency of the harvesting. But one way or another, an extremely hot planet is very unlikely to harbor an advanced civilization.

Theoretically a civilization that could capture 100% of the energy from it's star would be able to use however much of that energy it wanted to heat or cool the planet. They could make the conditions perfect for them. There could be a reason why they'd want the surface to be extremely hot.

This might be just the powerplant and they could be living somewhere else. A gas giant seems improbable as an advanced civ homeworld anyway.

There are no other discovered planets in the system, but perhaps there are some, or at least moons.