Cheap front shock seizing question

Trek 7100 hybrid, the front shock is getting stuck in the compressed
state. I really don't care for the shock value, but it prevents me
from attaching my fender when it's wet out. I was able to remove the
rubber plugs at the top about a year ago but couldn't figure out how
to get any further, so I shot some oil down into the mess and it was
better for a while.

But now it's seized again. This article
http://www.bikewebsite.com/suspension.htm

give general advice on fixing the suspension, but I don't have
replacement seals or even know if this is air or oil dampened.

Any pointers appreciated.