No idea if Hecklinski is the real deal, but regarding QB height, here is the distribution of heights for the NCAA QB among the top 50 in QBR:
View attachment 11708
The most common QB heights are 6'2" - 6'3"...you are much more likely to find a successful QB who is under 6'2" (18 instances among the top 50) than you are to find one who is greater than 6'3" (5 instances). That just stands to reason, there are a lot more people walking around who are under 6'2" than are 6'4"+...what are the odds you find someone who is both 99th percentile height AND has all of the necessary attributes to be a DI signal-caller? Tiny Venn diagram, there.
Regarding the last 2 NFL drafts, for the 22 QBs I could find combine info on, the median height was 6' 2.25", and 7 QBs were 6'2" and under, with 15 being over 6'2".
Hecklinski is listed as 5'11", so he would be a pretty extreme outlier to find success as a P5 QB, but not without precedent (Bryce Young and Kyler Murray are both under 6 feet). Currently, Utah, FSU, Washington, and Arizona have QBs who are under 6 feet.