The source for that statement is NACAC’s 2019 State of College Admission report (linked in the article).
Table 7 on Page 15 contains the ranking of admission factors based on an N of 220ish…and it does have demonstrated interest ranked ahead of those other factors.
Do you have any reason to doubt the veracity of that data? I think there are about 1,200 NACAC member colleges, so not a huge sample, but for this particular sample that’s what the data show.