it is normal for the starter IN post to  “appear” negative because the other side  of the  starter armature   is in fact  connected to the engine frame.  This is no diffeent than common light bulbs in the house… One side is ALWAYS on  white neutral (ground)  and black  turns it on from the wall switch  (solenoid  B+ red in outboards)

the point here is that if you apply B+  to the “seamingly grounded in post”… it will cause current flow into the starter  and spin the starter.

if it does  not the starter itself is jammed phusically and will cause  wires to overheat…. quite normal.

not clear what you did  I used a patch cable to jump over the solenoid – positive in to positive out. but if you connected the solenoid  main B+ in post from the battery ….  manually to the solenoid  B+ out post to the starter with a small gauge wire off course it will heat up due the   high amperage load  consumed by the starter.

SO…the question here is .. does the starter spin or not  if you apply B+ off the battery  directly on the starter B+ in post.