There's an argument to be made that, so long as your testing fully encompasses all visitors to your site, you aren't sampling the population, you're fully observing it, and statistical significance is irrelevant.
It's not unreasonable to assume it's a sample, I just don't think it's worth getting paralyzed by worrying about whether or not you have power, or getting into hacky tricks to try to fix it.
...but most power calculations are also sort of bullshit.