39:53-54 and the limits of forgiveness

[Seraphina]

Assalamu alaikum everyone, especially brother Joseph,
I hope you are in good health inshallah
There was this post of brother Joseph in fb a few days ago regarding the verses 53 and 54 of surah Az-Zumar
https://web.facebook.com/joseph.a.islam/posts/965397073597420 , i commented a question and he stated that he'd be happy to discuss it further in the forum, rather than in post comments
What made me worried and a bit confused was on of brother Joseph's answers regarding the impossibility of reaching God's forgiveness after sealing of the hearts, even if you repent afterwards. I quote a part of his answer that i'm talking about: "If God has sealed the heart of a disbeliever IN THEIR LIFETIME because proof was completed on them and they REJECTED IT, they will arguably not be subject to God's mercy even if they continue to live long lives and accept the truth later."
Can a truth still enter a heart sealed by God? Even if it does, how can God not forgive them if they repented during their lifetime? To me it just doesn't resonate with his mercy and justice. If we take a look in the glorious Quran for example, God says 'dislikes'/'doesnt love'. The word ''God hates'' is only used for one kind of sinners: hypocrites, i.e the ones who know the truth and secretly plot against it. Yet, He states that if they repent He will forgive them.
Can you please elaborate it further brother Joseph?
Any input from other members is highly appreciated!
May Allah forgive us all,
Seraphina