If a husband is excessive in his discipline, it is called cruelty. Cruelty may or may not be mortal. A person can be cruel to anyone, wife or not. Various factors go into deciding whether it is mortal or not.
There is no particular reason why it should always be a mortal sin for a wife, and not for other people or animals.
Cruelty towards a wife is an especially bad form of cruelty for many reasons.
It is an abuse of the husband's authority. It is a violation of his duty to love her. It is a contradiction of his duty to protect her. It is a failure in his duty to honour her. In most cases, he is significantly stronger than she, so it is bullying. Because he is her spiritual head, his cruelty also harms her spiritually possibly to the point of endangering her soul. Cruelty to a wife is never merely cruelty but brings with it a host of other serious sins. This is why it is virtually always grave matter and likely to be a mortal sin.
But the anonymous proponent's (henceforth AP) attempt to undermine the teaching that corporal punishment of a wife is only permitted on the condition that it is moderate is part of a pattern. At every point he distorts, contradicts, or undermines the Church teaching.
St. Alphonus Ligouri (SAL): It is a sin to maltreat a wife by "beating her, slapping her face, or calling her insulting names."
AP: It is only a venial sin. Don't worry about it. If a wife suffers, that's just life.
SAL: A wife can be chastised by her husband (i.e. permission)
AP: A husband has a duty to use corporal punishment on his wife (i.e. obligation)
SAL: if there is just cause, particularly adultery and
AP: whenever in his opinion she commits a serious sin
SAL: if she is verbally correctly several times but it does not work and
AP: let's pretend this part isn't there
SAL: if done in moderation.
AP: he should beat her until she bleeds
SAL: It is not lawful to use corporal punishment for trifling defects
AP: There is no objective basis for determining what is trifling and serious; it is up to the opinion of the husband. In effect, he may beat her whenever he wants.
AP: Don't worry about meeting SAL's conditions properly. It is only a venial sin if you make a mistake. The important thing is that you must beat her. Make sure you do it, no matter what. Even if it is illegal. Even if it will destroy your life and lead to your children being raised by a single mother.
Any intellectually honest person can see that AP's position is significantly different from the Catholic teaching of St. Alphonsus. The only support for AP comes from men who like the idea of beating women so much that will accept any excuse to justify it. We can only speculate about what causes them to be like this. For that matter, we can only speculate how many of them there actually are, since none of them dares to use his name. I would like to think that there are only one or two of them taking this intellectually and morally deranged position.
While I do not expect AP to acknowledge that his ideas are not Catholic teaching, it is not a good use of my time to respond to every post. I am going to switch to occasional summaries of his false and misleading statements.