To be fair
Grimnir098 clues and evidence are not the same thing.
Clues are indirect hints that may turn out to be evidence after being proven, whilst evidence is concrete/factual/proof.
That's not true, plenty of evidence used in court cannot be completely verified, such as eyewitness testimony. This is why the definition is "...facts
or information..."
Even if I concede your point, though, is Aglaecwif's understanding of Ella's character not evidence? Even if that wouldn't be accepted in a court of law, it's clearly presented by WW as factual, narratively speaking.
If it was wrong:
-Why would Aglaecwif give that information?
-Why would she be so confident in it?
-Why would that information be so valuable?
-Why would MC comment on its importance?
Thinking about this from a Doylist perspective, all signposts indicate Aglaecwif is correct.