> a line which was more appropriate in a era before the majority of novels had sequels

Serialisation pre-dates the novel.

There is a legitimate debate around the degree to which an author can amend canon with sequels. Various fandoms reject sequels in literature and film, as is their right. I think the broad consensus tends towards treating sequels as canon, however.

> For a recent controversy in this area, ask Google "Is Dumbledore gay?"

Did Rowling write a sequel with this? I thought it was scrawled into the margin of a screenplay.

I think you have to reject the text of the books to argue Harry Potter is an alien. You don’t necessarily to accept or reject Dumbledore’s sexuality.