It look like Diesel is finally seeing who loves him. Ein loves him so much, and sees him the same way in either form, and wants to protect him. I think now Diesel is now starting to see who loves and cares and who is using him.
I agree, i think Diesel is picling up on the subtle difference. Yanek is a well written character, I enjoy him in the story. He's obsessed with Diesel for now, like a possession. But I don't think would have ever really sacrafice his life for him like Ein will for him (spending a lifew in hiding).
It look like Diesel is finally seeing who loves him. Ein loves him so much, and sees him the same way in either form, and wants to protect him. I think now Diesel is now starting to see who loves and cares and who is using him.