That's close, but not entirely true. Some apple trees are triploid, like Gravenstein and Jonagold, but most are diploid, so not really polyploid. Apple seeds will grow just fine, but the reason they are grafted is because they are very heterozygous, and as such, any seedlings will not have the same genetic characteristics as the original parent apples, and in all likelihood will be inferior. When people breed apple trees, they can go through thousands of seedlings only to find one tree with superior fruit. By grafting, you keep the superior genetics of an exceptional fruit, like Honeycrisp. Most fruit crops are reproduced asexually in some way for this reason, with the exception of cantaloupe, watermelon, and papaya, which have much shorter lifespans, and as such are much easier to work with. Trees are also grafted because, by using mature plant material, the tree will come to bearing faster, and you can select rootstock that offers dwarfing and disease resistance traits, which are useful.
You are right that he was against grafting though, proclaiming that it was wicked, damaging, and against the will of God. Unfortunately, judging by the modern opposition to GMOs, humanity did not learn anything from his silliness. Today, we have opposition to the Arctic apples, which hopefully will be released soon, which have the relatively simple trait of non-browning. Anti-GMO people claim they are worried that GMO apples will cross pollinate other apples, despite the simple fact that apples are asexually propagated. That's right, these folks don't know the first, most basic things about apple biology, but damn it they're going to pound in their stupid point anyway no matter how wrong they are. Ridiculous.