I believe when they say it measures surfaces, they mean opaque objects. The suggestion was that you can't use it to accurately measure the temperature of the human body, for example, since it will only measure the temp of the skin the laser hits.
But transparent liquids such as wax are different since the laser can go through it. I don't know how accurate it is with liquids, and I am finding some variation. I think to some extent it depends on the container the liquid is in - if is it shiny, will might reflect the laser. And perhaps how deep the liquid is might be another factor. Just guessing here.
It's been about two months since I've had mine. In that time I've read of a number of people using them successfully with wax. But I think your suggestion to do a side by side comparison in the same batch of wax would be both easy, interesting, and set your mind at ease. I have found that the readings in my wax seem more stable when pointed at melted wax that is deeper. Since at times I don't melt large quantities, and my pan is shiny. I will tilt the pan for greater depth of wax and take my readings in the deeper liquid.
If you have been using one of these for along time, and your candles turn out good and are consistent, there shouldn't be anything to worry about.
Oh, and the laser thermometer makes a great cat toy.