I imagined it would be relative to wind speed at the craft, as measured by some instrument. Which would make the comic at least true in a general sense, as it does state the altitude is constant?

That is the misconception I am trying to address. Check out this wind report:

1kt of wind on the surface, 5kts at 100ft. My balloon is 120ft tall. My balloon is experiencing those 5kt winds while I’m floating inches off the ground. If I have an airspeed indicator in the basket, it’s going to be reading 4kts. (Actually closer to 5kts due to the change in direction as well as speed)

How about if I’m at 150ft in the basket, in 5kt winds. The top of my balloon is 270ft, in 12kt winds. My airspeed indicator is going to be reading 7kts. I’m going to have 7kts of wind in my face.

I’m trying to point out that the height of a balloon is very often larger than the gradient between two different air masses.

I have experienced 15kt shears: my basket is hanging in an air current 15kts slower than the air current that my envelope is riding within. I have felt 15kt winds in my face while riding in a balloon that is carried by the wind.