The science denier labeled is Ad hominem (personal attack) that harkens back to Holocaust deniers, who were Nazi sympathizers that denied the reality of the Holocaust despite the overwhelming evidence. So basically, the use of the label is comparing creationists and Nazi sympathizers.
The notion behind this label is that the person using it thinks that a particular theory is an irrefutable science and that consequently, anyone who disagrees with it is simply denying it. This ignores the fact that there can be legitimate reasons for questioning even well-established scientific principles. One possibility is that the person to which they are applying the label has not had the concept presented to them properly. Another legitimate reason would be they do not trust the concept because of who is pushing the idea, climate change is a good example of this. The third legitimate reason is that they see possible flaws in the idea. Whether or not they are correct is a different matter, but you are not going to convince someone that they are mistaken about something by calling them names.
This is a common problem with evolutionists, they will often resort to name calling of one sort or another. This is a sure sign of not being able to actually answer the objections. When they think they can answer an objection they will, however inevitably evolutionists will resort to some former name calling and science denier is one of them.
One of the big problems with using this derogatory label is a misunderstanding of creationists. Besides the fact that this label compares creationists to Nazi sympathizers, it also implies an irrational reason for not accepting evolutionary theory. This is despite the fact that there are plenty of rational reasons for rejecting universal common descent evolution. The number one reason is that its mechanism has never been demonstrated to be capable of doing the job. In order for universal common dissent to be possible, it needs a way of producing new complex specific information. Organisms need to somehow acquire the information necessary to produce hearts, lungs, two types of blood cells, wings, and many other structures, including many of the complex structures that are found in every living cell. Before you can claim universal common descent is a viable explanation for the diversity of life you need to demonstrate that it is a possible explanation for the diversity of life. Without what adds up to gigabytes upon gigabytes of new complex specific information, not duplications, and not degenerative mutations, but completely new information capable of producing complex structures that did not exist previously. This is just one of many highly rational reasons for not accepting universal common descent evolution.
Another aspect of this derogatory term some evolutionists have stopped seeing science as the theory and the data, but actually give evidence of seeing themselves as the science. This attitude is evident from Dr. Fauci of COVID-19 fame when I asked about the science, he replied that he was the science. Even if a statement like this was intended to be tongue in cheek it still shows how they see their relationship to science.
Ultimately, the use of such a pejorative shows an unwillingness and even inability to deal with disagreement. If evolutionists were as objective and open-minded as they claim, they were not used such pejoratives. They would be willing to hear people out who disagree with them and try to understand their position. However, their reaction to both creation science and intelligent design shows an attitude of arrogance that even if they were right would prevent them from learning anything from their opponents.