I think a lot of this boils down to the distribution you install, what kind of background services it runs, and how effective its energy tunings are.
As an example, IMO an idle computer should have all CPU save one or two at 0% utilization, and that remaining CPU(s) shouldn't be averaging more than a few percent, in short spiky bursts. FreeBSD or Debian are like this, but Ubuntu is not.
Yes, saying that a laptop runs "Linux" does not provide any useful information when talking about battery life.
I use Gentoo on a Dell Precision laptop and I do not see any battery lifetime difference between it and Windows.
However, I do not doubt that with other Linux distributions or with a Gentoo that has a very different configuration, the results would not be the same.
I think that it is very wrong to say that a laptop with Linux has a worse battery life than one with Windows, but it is right to say that in many cases a laptop with Linux needs an experienced user to configure it properly, in order to have the same battery life that it would have with Windows out of the box.
On the other hand, when installing Windows 10 Enterprise on embedded computers, I have encountered many cases when Linux had great performance in a default installation, while with Windows 10 I had to waste many days with tuning, e.g. with discovering that certain services must be disabled, until obtaining an acceptable performance.
As an example, IMO an idle computer should have all CPU save one or two at 0% utilization, and that remaining CPU(s) shouldn't be averaging more than a few percent, in short spiky bursts. FreeBSD or Debian are like this, but Ubuntu is not.