The exact value of the analog reference only matters for printing the voltage. As MartinX has reiterated, for calculating the temperature the voltage is irrelevant. Go ahead and change 4.86 to 9837.21 in the sketch. You'll get the same temperature.
It's kind of a sloppy, blog-like tutorial aimed at a beginner audience. But the code actually works reasonably well.
I've used the Steinhart-Hart equation for thermistors. Over a limited range it can be quite good.