So through discussions with others, I have confirmed this information is wrong, at least generally, but that doesn't change that I was told that twice, and that the Internet Help Desk was unable to fix the problem (escalated it to someone else on the back end who was also unable to force fix it - they couldn't figure it out).
The issue is not the number of days that can be reserved for AP with a resort stay - the first people told me that is exactly what Passholders get with a resort stay - the ability to book more days. The problem is that I was told, and there is some language on the site that could be interpreted to back this up (other language that arguably completely contradicts it, however), that while Passholders get those additional days, our Park Pass reservations will be pulled from the Passholder pool, not the Disney Resort Guest pool. I was told that pool is only for Resort Guests with tickets, not APs.
Some on the DVC Fan page were able to confirm based on days on which the only availability is for Resort Guests that they are able to reserve a park with their AP. However, this does not mean they aren't doing something like this for the 50th - since that's where I'm having the problem. On 10/1/2021, the calendar shows no availability for Passholders, only for Resort Guests and Theme Park Ticket Guests, and I am unable to book for that date, despite having a resort reservation. I'm able to book for any other days of that trip, but not at MK for 10/1. Internet Help Desk was not able to fix this either, so there is something, potentially, going on. We know there were some questionable things going on with resort and DVC reservations/availability at one point, so who knows.