Do you have any that state this?
https://www.bustle.com/p/what-does-the-plus-in-lgbtqia-mean-...
Though I'll admit the contentiousness of this designation, I don't think the intent of "+" was to include incest.
After reading that article that in various places calls out...
- "The plus is widely taken as a symbol to represent self-identifying members of the community who are not included in the LGBTQIA acronym"
- "The plus in LGBTQIA+ not only represents other sexual labels and identifiers, but also the experiences of those within the community."
besides the quote you already mentioned which includes the weasely "Some say", I personally don't really see as a strong of a consensus as your first comment suggests, but appreciate the perspective.
1) the acronym can only get so long because it becomes alphabet soup.
2) the default posture is to ally with groups that haven't been included yet.