When you do the threshold step, there is no antialiasing (smoothing) around the black or white edges. You can see a close-up in the inset.
You can smooth up those edges by doing this:
Use the color select tool and select the black in the image.
Select > To Path
Unselect
Fill the black and white image with white.
Select > From Path and fill the selection with black.
You have smoother edges now.
The inset has a zoomed in area of the image.
Do NOT gaussian blur the results by 2. They don't need it now.
Continue the tutorial at the plasma layer.
After you have merged the plasma layer down to the blur layer, you can stroke the path and make the black lines thicker if you want.
I chose the Circle (01) brush and changed the size to 1.50, FG color Black.
Edit > Stroke Path. I chose Stroke with Paint tool and stroked it with the Paintbrush.
You can see the difference in the results. You can use a larger/smaller brush if you wish.
I sent an email to the author of the Tie-Dyed Cows tutorial and gave him a link to the tutorial in here. He said the tutorial is over 10 years old now.