Am I sinful if I incline towards the opinion of Ibn Taymiyyah and Ibn Baz—may Allah have mercy on them—regarding one who abandoned prayer and fasting and then repented, that repentance suffices and he is not required to make up what he missed?

This is a matter of disagreement among scholars. If you are qualified to weigh scholarly opinions and you find a particular, well-supported opinion to be stronger and issue a fatwa based on it, then there is no sin upon you. However, if you are a layperson, then follow a scholar whom you trust. There is no harm in conveying both opinions to the questioner and informing them that you trust the one who rules that making up the prayers is not obligatory. Nevertheless, the more cautious approach, to avoid disagreement, is to make up those prayers.