You are testing temperatures far out of the range of most IR guns that people will buy.You can very easily test this yourself in a manner of 5 minutes and come to the conclusion that your statement is false. Put a pan on the stove and set the burner/element to low-mid. Let the pan heat up. Take a reading with your e value set to as low as your device allows (My gun allows for a low of .10 E.) record the temp. Set your gun to it's highest E value (mine is 1.00) record the value.
Because it's such an easy experiment, I decided to do it too (still, feel free to test it yourself). With my IR gun set to an E value of .10, the pan read 960F. 3 seconds later after adjusting my E value to 1.00, the pan read 230F.
The point is, IR guns need to be calibrated to read the material they are being aimed at. The inaccuracies of IR gun's due to emissivity is greatly compounded when the surface material is reflective/shiny/etc., such as in this case when the OP is attempting to measure his DIY rock which was coated in a protective coating such as polyurethane, which adds a sheen to the surface.. Sure, the inaccuracy may not be as apparent when it's something more organic such as a log, because most wood types are near the .95 E value.
Digital with probe end thermometers don't suffer this issue. IR guns do. There is no arguing that fact.
Both thermometers can have a place on a hobbyist's tool belt.
When we are talking about lower temperatures found in a viv even setting an incorrect e value will be minimal because we are not talking large numbers here. Temperature varience is going to be a lot more when using number's in the 100's comapred to numbers in the 10's of degrees. Most people use one of three things - rock, wood or some sort of cloth based hammock. All of these fall in the high 80's to mid 90's on the emissive scale. Even reading something thats .80 on the emissive scale would record a temp that is accurate to within less than 1 degree on a IR gun set to 0.95
Can't say I've encountered many people who use a shiny polyurethane coating on any fixtures. In fact its recommended to use a matte sealant on home made artificial rocks, etc.... a shiny one can result in burnt retpiles due to heat reflection off the shiny surface.....
IR thermometers are specifically designed to record surface temperatures and give you a reading (normally within a second) and can update that reading every second usually. Digital thermometers with rubber / plastic probes often sold in the reptile trade measure ambient air temperatures and do not measure surface temperature - they are just measuring the temperature of the air being heated up by the emitted heat. They also require long time periods to even start recording a temperature close to accurate - usually 5 minutes or more and take considerably longer to record variences.
The surface temperature of the material emitting the heat can be significantly higher than the ambient air temperature being heated by that material. Put your hand above a bulb dome reflector and feel the temperature of the air then put your finger on the dome itself and see how hot it is. Thought I'd use that example seeing as we are using extreme examples.
Probe thermometers are perfect for monitoring the temperature gradient of the air within a vivarium but, in my personal opinion, when it comes to measuring actual surface temperature they are inferior to IR thermometers which are specifically designed to measure surface temps.
Like I said each to their own when it comes to how people wish to record their basking spot temps and people should make sure they are fully versed in the correct use of any equipment they use for their pets. Otherwise its like buying a T5 UV and installing it 4 inches above your basking spot!