Eren didn't forget to say thanks to Jiana and Levine for not spying on his breakthrough.
Getting to expert rank and beyond was a very personal thing. Most rankers would only consciously attempt to achieve their breakthroughs in seclusion. This was because of the world phenomenon they created around themselves.
A ranker's entire path would be manifested at the time of a breakthrough into Expert rank and beyond. Someone could learn a lot about a ranker, their strengths and weaknesses by observing their breakthroughs closely.
The three ladies would have been shocked beyond belief if they had seen the myriad of elements and their Aspects he was cultivating. They couldn't have kept track of the number of runes he had absorbed into him. And they would have doubted their sanity if they had realized that Eren also had two distinct classes he was progressing in at the same time.
