Originally posted by TheHulkster
In his diminished state, he most definitely lacks the power to face Thane. He does gain power beyond that diminished state that allows him to fight Thane, but it is not stated that this gain takes him beyond where he was before the sickness. The dialogue supports that it doesn't. This may change in the future, but at present, it appears that he is back normal.
Or simply Thane is a far weaker host than the previous one.
An ideal host for the Phoenix is usually a very strong psionic.