I am sure there is a day of the week that statistically has more arrivals than others. I just think the number is so very small it's not enough to have any impact on anything related to getting your FP, ADR, anything.
Not sure how long ago your brother managed an on site resort but in the past 3 years we've sure never had trouble finding a room available no matter what day we wanted to arrive. I've never had to change our trip plans because there were no rooms available.
I'd dare anyone to hang around a Disney lobby for any length of time and not see guests checking in and out all the time, every time you are there. I know I have. Just look at home many ME buses you see coming and going, and that's just for air travel.
Tons has changed at Disney since 2007 so yeah, I'd say that's outdated info, if it ever was accurate.
Even the "don't go to MK on a Monday" has changed. Crowd patterns in general have changed just in the past 3 years and one of them is related to the fact that people now arrive every day of the week.
Sunday wastes half the Weekend because you wasted Saturday, half of your weekend. We always arrive so we spend both of our "free" days off at Disney, reducing the number of vacation days we have to take. I sure don't want to spend one of my free vacation days at home and add more work vacation days that I need to take. That doesn't make any sense to me. But I guess I'm alone in that
DH and I do our trips so we arrive either Thursday night after work or Friday morning (depends on when the flight is cheaper) and then come home last flight out Tuesday night. That way we are only taking 3 days off work and get 5 full days at Disney. When we bring DGD we add a week to that and come home the following Monday on the last flight out and we end up only taking 7 days off work, 6 if we can make it over a Holiday like we did with Memorial Day this year. The fewer days off each trip equals more trips we get to take, win-win for us