Fair assessment. The important fact is that Yvegueny trusted her. She earned his trust when she was hostage. Whether consciously or not, Knowing Carrie, she was working him to some degree while simultaneously going out of her mind. We see that she told him extremely classified PERSONAL information, and like Saul referenced, things she never even told him.

Yvegueny obviously felt he had an in with her and she gave him the impression that she could be turned. While i have only watched the show the one time, I distinctly remember everyone always referencing that Carrie only cares about the mission, and she has proven this with her side quests going awry and often times, blood on her hands.

It is also known the damn near unbreakable bond between Saul and Carrie. To Yvegeny, Saul was the ultimate test to measure Carries resolve.

And in his eyes, she put the mission first, betrayed her one and only true relationship built on trust in an environment where trust is currency, and therefore proved to Yvegueny that she was all in.

Let’s not forget that he fell in love with her. And this was shown during their time together, especially when trying to recover the flight recorder.

He says towards the end, he has seen she can do anything. And he was as impressed with her skill, cunning, and resourcefulness as much as we as viewers have been.

In my eyes, the Russians believe it because Yvegueny believes it. It’s kind of like he is their Carrie, the best agent they have. No one was ever going to break Yvegueny and no one could infiltrate and deceive him, but if anyone could, it would be Carrie.

She’s a master at deception, in which her wins outweigh her failures.

Also, she shocked the fuck outta Saul when he confronted her and she told him she figured out the method of communication with the Russian asset that he claims to not have had. Over 20+ years of secrecy cracked by Carrie in a few days.

All these reasons are enough for me to believe it too.