First, a word about the website. This is presumably based either on 2018 data or historical data through 2018, though they haven’t made any of that clear. Those of us who followed the career placement PDF’s in past years will recognize the wording. Most likely the calculations are the same; however, that’s small comfort to those who have issues with those prior placement reports.
“Substantive” plans mean a job or grad school and has since I’ve been following career placement at UChicago. Here’s prior wording (from 2017 in this case): " 93% of the Class of 2017 has substantive post-graduation plans. These plans include working at leading organizations throughout the United States and abroad, continuing their studies at some of the world’s most respected graduate and professional schools, starting their own businesses, and serving the greater good through organizations like Teach for America."
The good news is that a higher percentage of UChicago grads are graduating with plans in place. As a parent, that’s the most meaningful statistic - and my guess is that Nondorf wants to KISS. He’s not building these marketing pitches for all of you old grumpies and your perpetual states of confusion and flummox 
The medical school wording is also an update from the earlier reports; 2017 reads as follows: “85% of UChicago 2017 cycle applicants were admitted to medical school, more than double the national average of 41%” Of course, BlueBayou is on record maintaining that colleges cook the stats for med school.
I have older kids who are graduated from or finishing up undergraduate professional degree programs, and skepticism regarding placement stats abound there as well. It’s a universal phenomenon.
I’ve been tracking LS acceptance rates at UC for a couple of years. Not so concerned with how they are calculating the data. You can only BS so much before your bluff is called. Anyway, here is the history I have:
85% of UChicago 2015 cycle applicants were admitted top 14 law schools
84% of UChicago 2016 cycle applicants were admitted to top-14 law schools
84% of UChicago 2017 cycle applicants were admitted to a top-20 law school
83%-88% Top 15 law school acceptance rate for UChicago students
They seem fixated on a percentage somewhere in the 80’s (perhaps someone learned in marketing class this this is a magic number) but the quality of the law schools seems to go up and down depending on the applicant cycle (which will include 4th years and alums who are usually 1-2 years out). Unlike BlueBayou, I’m not so very concerned about what “T14” or “T15” means because that tends to be jargon for a set of schools in the GULS range and higher. T20 I’m a bit less comfortable with.
I do NOT believe these stats are ‘pie in the sky’ or somehow misrepresentative - I’m betting they are accurate. But law school is so very dependent on other factors, including demographics (as has been pointed out) and gender, that they may simply not be applicable to my kid - or yours. They are a helpful metric to gauge the quality of the students at the College who are applying to law school, and not much more than that. Anyone looking at these stats and thinking they are set for a T15 will probably be about as happy at the end of the application cycle as someone who thinks applying ED means “sure thing” for the College. The stats over-simplify a LOT of other factors.