Quite the contrary.
She thought he was another failed copy. She acknowledges he still might be, but no longer believes that he is.
"You are not who I thought you were at first". If he was blank slate, it would have been too early to tell, since it's a developmental process. She straight up differentiate between "you" and "not you".
Look, fact that he has moments of lucidity, occasional slips of a tongue, as well as people like Maya and Ayane noting that he seem to be plating dumb intentionally would indicate that there IS a fully formed personality out there, as opposed to one he can develop into. And, again, Sensei at first insist that he was "isekai'd". There is something different going on there.
Think about what we know about OG sensei (tutor turned teacher, loves poetry, took his job very seriously). Then think about the first day in class. Of course she wrote him off as a failed copy. "Do what you want, I won't teach anything" is about as far as you can get from his original work ethic as you can get.
Right. Again -- we're forming a personality on a newly blanked slate.
Take a chalkboard at the end of the day in a classroom and erase everything on it. Now try to reconstruct what was on it. Will it be exactly the same? Probably not. Handwriting may be slightly different, positioning will change. Even if all the text is same in terms of content, it's going to be different because of semi-random variations in how the i's are dotted and the t's are crossed.
Erase it and try again. And again. And again. Under the infinite monkeys theorem (weeb note: If you put an infinite number of monkeys in a room filled with an infinite number of typewriters and give them an ininite amount of time, one of them will invariably write the complete works of Shakespeare), if you repeat this process enough times, you'll eventually hit an exact match.
That's the process that Sensei has been going through since the jump. He wakes up, no memory, no idea where is or even who he is. He begins forming a personality. It's imperfect. Eventually, during a reset, he gets erased and the process starts again. Maya's been doing this a VERY long time, and she now believes that THIS Sensei is the closest she's ever gotten to a complete match of the original. That doesn't mean those other Sensei's weren't senseis. They were just imperfect attempts at reproducing his personality, not other people entirely. All the parts were there, the i's were just dotted wrong.
You realise there is an inherent contradiction here? If it's a process of becoming Sensei, why would she dismiss him right off the bat just because he behaves certain way and start treating him the way she does? She mentions that if she knew from the start who he was, their interactions would have been different, but now that ship has sailed (can't find the even when she says this, though).
"No one ever remembers" about what led them into reincarnation. She could be talking out of her ass, or talking about previous experiences with other people in Sensei's body.
PAY ATTENTION TO THIS ONE:
She straight up says she mistook him for someone else, didn't realise it was "him". He isn't changing, becoming more like Sensei she knew, he IS this guy, as opposed to previous iterations, where it was SOMEONE ELSE. And, since he plays dumb and has "amnisia", she could't tell if it's him or isn't anyway.
When this Sensei started to behave differently, she just thought that new guy is changing, adapting, not unlike like all the rest before him.
So now she's angry at her own stubbornness, since she's used to dismiss new Sensei every new cycle as another isekai'd person, not her guy.
She's saying this is the closest to her goal she's ever gotten.
If I remember correctly, she says that this Sensei have gotten the farthest. We still don't know what her actual goal is.
Again, to reiterate: everything seems to indicate that pervious Sensei's weren't the right person, an OG Sensei, a blank slate that just didn't make the cut, didn't have enough time to fully develop and recall who he once was. Not the case at all.
It was different people entirely, as far as Maya could tell. She couldn't tell right away and eventually just got used to it and have adopted a certain persona and an energy conserving lifestyle. This one IS OG and she didn't expect it, was caught off guard by his "amnesia".