That just means they don't argue as much on the friendship route.
Not really. The codex mentions, and quite clearly, that Anders have embraced the spirit within and they are one now. Keeping his calm, Anders only seems to lose control when fighting Templars. And in this path if your friendship is high enough and choose the right dialogue he will tell you he has been planning something for years to overthrow the chantry but he does not want to incriminate Hawke, so you see no one is pushing or controlling Anders. In friendship route he embraces the spirit within and he gains control, only losing himself when fighting Templars which is understandable given his anger towards the templars warped Justice in the first place.
Hawke's assessment of Anders's mental state shouldn't have any bearing on reality though... if he is one with Justice, that should always be true, regardless of what Hawke thinks or how well Hawke and Anders get along. Perhaps Justice simply does not attempt to interfere with Anders when he's acting according to the spirit's wishes... that doesn't mean that they are fully merged on the Friendship path but not on Rivalry. It means that on Rivalry, Justice is less tolerant of Anders's doubts and attempts to think for himself.
You say "shouldn't" whereas every single companion is affected by Hawke's assessments. The codex is very clear that getting the support he needs (either from a friend or lover who is Hawke) his will grows. Given that how important a mage's will is because they only requirement of becoming an abomination is wanting to become one I'd say moral support makes a huge difference here. They are fully merged on both paths, Anders's loses it on rivalry so Justice has to take over to finish their task whereas in friendship route no such thing happens and its actually Anders who has been planning to overthrow the chantry for years.
But still technically Anders is an abomination right? Faith was weakening by the time she revived Wynne.
If we are speaking technically both are abominations. So yes.