You're kind of correct on both guesses. You can get that change by changing the viscosity OR the airspeed.

He elaborates later on, but you're changing the Reynolds Number - a calculated value from the velocity, fluid density, viscosity and length. The cool thing about a Reynolds Number is that you get identical (in theory) airflow characteristics for two setups with the same Reynold's Number, even if e.g. the airspeed is different.

I think this is entirely intentional. All articles by Bartosz build up from simple basic principles and avoiding specific technical terms is a good way to onboard viewers of mixed backgrounds without scaring anyone off. Viscosity is actually mentioned for the first time only roughly three quarters into the whole thing.

From the HTML:

   <div class="slider_viscosity" id="fdm_hero_sl0">(...)

Also, it says "this substance", which I initially thought referred to the cube as it was just mentioned in the previous sentence. But I guess it's the "fluid".