Jung designated "Thinking" for a specific, impersonal way of judging, but all of the functions are thinking (cognition). In this quote, he is referring to the specific function of "Thinking", not to rational cognition in general, because Fe falls under rational cognition.
Feeling is also rational, because it reasons to form judgments/conclusions. Jung even calls Feeling logical, because it seeks a consistency. It does not just reason to form a singular conclusion that stands alone, but considers how all of the values connect & relate because they must make sense in relation to each other. An F-dom will not consciously hold two contradicting values (something P-dom maddeningly may do). The harmonizing of their values is something they spend a lot of time doing. When inner harmony is spoken of for F-dom, this is what is meant, not emotional calm.
"Feeling, like thinking, is a rational function, since values in general are assigned according to the laws of reason, just as concepts in general are formed according to these laws" - Jung
"Feeling is distinguished from affect by the fact that it produces no perceptible physical innervation, ie, neither more nor less than an ordinary thinking process." - Jung
"Once they (sensation and intuition) are distinguished from feeling, it becomes quite clear that feeling values and feeling judgments - indeed, feelings in general - are not only rational, but can also be seen as logical, consistent, and [as] discriminating as thinking." - Jung
No, the evaluations are made with a consideration of what emotions signal, but not based solely on them. Emotions can be wholly dismissed as well.
I will reiterate that Thinking types tend to project their own inferior Feeling onto F-doms, and being inferior, it is less differentiated from emotion. Dominant Feeling is as purely rational as any thought process can be. Of course an individual's maturity, intelligence, etc, always comes into play.