Thief God Milton did have his fair share of strange experiences, so he wasn't too worried to have encountered a spirit like this. As long as he was using his own spirit to assimilate this spirit, he believed that everything strange that just happened would just explain itself.
In terms of the ridiculously high concentration of the druid spirit, for instance, to his understanding, he never remembered any class holder who had stronger spirituality than the divine spirit. That being said, it was very weird that the spirit didn't take any damage under the strikes of his golden spirit. He couldn't assimilate it, much less take any control. The soul he was encountering was like a fort. His golden spirit could not attack from the outside and never go into the centremost part of the spirit.