You're right, the answer depends on who you ask.
Native support for the Nest thermostat is not available, yet. That is, you cannot say, "Alexa, set temperature to 78 degrees."
There is a workaorund through IFTTT. It works, but it's not ideal. For it to work, it has to be triggered by some kind of event, such as adding something to your to-do list, playing an mp3 song, etc. Once that trigger has been detected, only one predefined command can be issued. So let's say your predefined command was to set the temperature to 72 degrees. You can say something like, "Alexa, add Nest to my to-do list." Then Echo will set the temperature to the predefined value.
Then there are the hacks. They are difficult to implement if you're not technically inclined. With some hacks, you can say something like, "Alexa, to-do temperature 80 degrees."