In the context of that particular field—computer science and engineering—it is likely that simply no qualified applicants were found, or the ones that were found got better offers elsewhere. This is very common in computing right now—it is a very in-demand field and finding high quality candidates who could succeed in getting tenure at Clemson, an R1 university, is just difficult. There are simply not that many high-caliber candidates.
Note that the position is in a department of computer engineering, but the qualifications (emphasis on AI/ML) clearly are looking for computer science.
The Taulbee Survey is put out by the Computing Research Association, and is the gold-standard industry publication about computing in academia. Among many other things, they report on reasons for unsuccessful faculty searches. If you look at Table F2 and F2a on pages 33–34 of the current survey, you will find number of successful searches and reasons why searches failed.
First, for US Public universities, tenure-track searches failed 25% of the time. Of those searches that failed, 67% failed because someone couldn't be gotten. Either because no qualified applicants were found (14%), or applicants accepted a better offer somewhere else (53%).
To your specific question—are any of these positions being listed with no real intention to fill them, the survey lists as only 5%—"Technically vacant, not filled for admin reasons."
I might also speculate—and I caution that this is wild speculation—that Clemson in particular might have trouble hiring in CS due to their research trajectory. Clemson moved from R2 to R1 status in 2015, which means that they will probably have a difficult hiring climate in their department. They are probably seeking to hire top-quality R1 research candidates, but they probably don't have as many mature CS researchers, CS research opportunities, and CS research support as other, more mature R1 programs. This would make it very difficult for them to compete for those top candidates. Essentially, they're in the process of moving from being a big fish in the R2 pond, to being a small fish in the R1 pond. As someone in a CS department at a university in a similar place, I can say that it has been incredibly difficult to hire, and our ability hire has not kept pace with the growth in our teaching programs, to say nothing of our university's research ambitions.