Well, I did some research and I found out that there is a bit of an inconsistency with the UT CRT of Dura Neg, especially with the Sans' part. Sans seems to be NO EXCEPTION when it comes to monsters limited durability, and I'll explain why.
First of all, I read about the CRT and the main reason why Sans is the exception is because people used to believe that, aside from the other monsters, there was no way for you to mitigate his damage by wearing conventional armor, but this seems to be a lie since, well, there IS a way for you to mitigate his poison effect with the Torn Notebook and Cloudy Glasses since these items combined increase your invulnerability frames, which I searched for in the wiki and they ARE considered part of durability, which means that THERE IS a way for you to tank Sans' KARMA poison a bit more with armor. And if you're able to do this, then that means Sans has NO TRUE DURA NEG, and just has a stronger level of limited durability negation compared to the other monsters. So I don't think we should keep Sans the exception anymore, which also means he should get back at having limited durability negation.