I don't think that is correct. I believe Haarms eligibility clock started while he was at his prep school in Kansas and that's why he enrolled early. He could have enrolled in August the following year but would have already burned a year of eligibility while at prep school.
Per his wikipedia page
"Haarms enrolled at Purdue a semester early and
redshirted the second half of the 2016-17 season because the NCAA ruled he would have lost a season of eligibility had he stayed at Sunrise Christian"
https://en.wikipedia.org/wiki/Matt_Haarms
So yes he could have theoretically enrolled at Purdue in August but he would have only had 3 years of eligibility remaining unless he redshirted that August which is exactly what I subscribed above. Obviously it made much more sense to enroll in January as he would have been behind the curve enrolling in August (not to mention already down a year of eligibility) so his hand was likely forced.