I mean- it's logical. It fits in the hole, has two 13mm bolts that hold it in.
On the MkII you need a special tool to get the bugger out....
My 91 Jetta needs a change as I think it has its original struts. I assembled a tool that worked on one side- a 13/16" spark plug wrench centered with a 7mm allen key driven by a 7mm box wrench. With wife and I pushing we got one side off with difficulty. Found that the split retainer nut had been driven into the top nut probably causing the struggle. Could not get the other side off.
I went to my buddy who has quantities of pipe (cheaters) and more strength. We proceeded to round out the center of the strut rod.
Now what? Any suggestions? My pal suggested I drill out the center,, but I am concerned that the spring retainer nut (the split nut) may be compromised that way. My thinking is to cut the top plate with a cutoff wheel, then take the strut out, compress lightly and cut the top off with the cutoff wheel. I have a whole strut assembly to use to replace this, but I would like to use the newer spring and spring retainer plate off my Jetta ( this part on the replacement strut I have is pretty rusty).
Because they suck.

The engine mount setup and front strut mounts are two things I have always disliked about A1's. The very early A1's had a neat setup that you can take apart and get poly inserts for.
Nylock nut?
Put some heat to it, until the nylon melts.
No, not a nylock nut

It's not corroded at all- just really tight against the split nut under it, most likely
An impact gun should get it off. If it won't by itself put vice grips on the strut shaft to hold it from spinning.
I think I have an impact gun somewhere that I never used- can't hurt to try that. I don't think vice grips are going to hold that shaft - it took a lot of torque to strip out the center of the shaft...
That's actually why VW changed the design... they hold the patent on the design for impact wrenches.

I'm generally not a bit fan of muscling stuff off, but in this case an impact wrench is the exact right tool when trying to loosen something that wants to counter-rotate, and it works a treat.