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.