Is saying "Cut off whoever wrongs you"—which was falsely attributed to God—considered lying against God, necessitating seeking forgiveness?

What you mentioned is speaking about Allah without knowledge and is contrary to the Sharia, which commanded maintaining ties of kinship even if they are severed. Allah's forgiveness for those who repent with sincere repentance is not too great for any sin, even polytheism. Whoever repents sincerely, their sin is forgiven, for Allah the Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Allah forgives the sins of those who repent and transforms their evil deeds into good ones. Therefore, you should think well of Allah and implore Him to forgive you and accept your repentance.