Character fandom shouldn't blind you to that character's faults.
Given that the Guardian's insight into every other companion proved accurate, that must also be the case with Leliana. She lied when she said the Maker had spoken to her in a vision.
"And you. Why do you say the Maker speaks to you, when all know that the Maker has left? He spoke only to Andraste. Do you believe yourself her equal?"
"I never said that, I..."
"In Orlais, you were someone. In Lothering, you feared you would lose yourself. Become a drab sister and disappear. When your brothers and sisters of the cloister criticized you for what you professed, you were hurt, but you also reveled in it. It made you special. You enjoyed the attention, even if it was negative"
"You're saying I made it up for, for the attention? I did not, I know what I believe!"
Interestingly, the Guardian doesn't seem to be questioning the validity of Leliana's claim. He's questioning Leliana's humility. He's intimating that she's not the humble servant of the Maker she claims to be. Whether she's crazy, lying, or genuinely hearing the Maker is irrelevant. He's questioning that she was using the claim to seek attention, rather than doing the Maker's work without that added detail.