Excellent point. We've been keeping temperature records across the globe since the late 1800's. Which is roughly the time the Little Ice Age bottomed out and we began yet another warming period (Modern Warm Period, see graph above). So the left gets away with saying highest temperatures "on record" in hopes that we don't realize that there were no temperature records of the prior warming periods (or that we forget the prior ones exist and thing the Modern Warm Period is unique).
I understand what you're saying, but arguably there ARE temperature records going back hundreds of millions of years. Ice cores etc. And THOSE can be scientifically determined within some error, but certainly indicative for warm/hot periods and ice eras (not ages) lasting millions of years.
In contrast, what will happen in the future, be it 10 years, 100 years or 100,000 years cannot be measured or studied by default, and is therefore unscientific. Computer models are not science.