Yes he is their child but only the Duke knows, Aisha doesn't remember and abel isn't aware as well. There's a chance that the brother also knows (but I don't remember well)
Then why would the duke say “only if he was my biological child “in the second last chap
I forgot the exact sentence but he did say smth along those lines
So, I went back to check. The Duke and aisha are talking in the scene you're referring and he tries to know if she remembers anything. She says she doesn't and speculate she might recognise the mother because she does know she's the one who entrusted the baby to the Duke (the part of her memory which has been wiped out was only about giving birth and the knowledge that he's theirs)
After making sure that aisha reeeeally doesn't remember, the Duke goes along what aisha says and agrees that he wish the kid was really theirs (even though its clear he knows he is but he definitely doesn't want to push the truth on aisha as she most likely will feel great guilt by not remembering)
Also there's a hint of doubt from the Duke, since he's the only on knowing the truth he questions himself about it being a hallucination but it was brushed off right after
Wait I am confused I thought Abel is their biological child? In the flashback pretty sure, it was past Aisha who gave Abel to ml no? I def thought he was their child maybe not them currently but still like them of past??