University scale tunnel testing quickie
 

I guess you have to know what to look for, because it just looks like semi turbulent smoke over a tiny model to me.
So they were actually able to attain the cD of the vehicle using such a tiny tunnel ?
( That is what I understood from the video )

All the action was within a fraction of an inch. Obscured by cloud cover.

Maybe they were circulating Nitrogen or Argon to compensate for the Reynolds number?

Looked like water vapor to me. Like Cd said, hard to make out exactly what they are domonstrating or measuring

That is way too turbulent, they ran smoke to check for turbulence, there is no way that you can tell if the model was causing the turbulence or whether the fan/flow straighteners or lack thereof caused the drag.

Are DIY wind tunnels and 1:18 models connected to a load cell a viable test? Or is it not possible to get the wind speed high enough?

As I understand it, model scale adds some intricacy which is not linear to full size.

attain
 

It'd that 'verisimilitude' ( dynamic similarity ), supercritical Reynolds number requirement in play.
At an average 15-mph @ full-scale, they can maintain a laminar boundary layer, up to the maximum body cross-section, which qualifies it as a 'laminar' body, of very low drag.
Scaled up for a family of five, I don't think they could expect numbers anywhere close to Cd 0.0512.
Like Aptera and the 1978 CNR car.