The Moms Panel response you linked actually says the opposite of what you are saying, so I'm not sure why you linked it. Neither of these sources is really reliable though. This has been discussed repeatedly in past Rider Swap threads where several people have posted their emails, chats, and phone conversations with Disney about Rider Swap and gotten completely different answers. The problem is that the personnel who communicate with guests through these means are often misinformed and have no idea what they are talking about or how things are actually handled in the parks-- it's like calling the Front Desk from your resort. That person isn't actually at the front desk, and likewise, the people emailing, calling, etc in Guest Services aren't actually ride CMs with direct training on how rider swap actually works and is to be enforced in the parks. As for the Moms Panel, they are notorious for giving contradictory or incorrect information, and there's even a statement at the bottom of their page that says, "These views are not intended to reflect the opinions of Disney Destinations, its affiliates, its management, its agents, or its employees and have not been reviewed or approved by Disney Destinations prior to posting."
That being said, there have been a couple reports on here of people being told at certain rides that everyone should have FP, but there have also been numerous reports that FP was not needed for Group 2. Frankly, whether there is an official policy or not is unclear given that others have received communication from Disney that says Group 2 doesn't need FP and Disney has yet to post anything official on their website.
Lastly, one of the advertised perks of rider swap is that the adult waiting behind with the too short child doesn't have to ride alone when it is their turn. For instance, Dad and Kid A have FP and ride while Mom waits with Kid B. If Mom only has a FP to use when Dad and Kid A return, then she has to ride alone. Having a rider swap allows her to ride with kid A. Also, when riding things like Soarin and FOP with Pre-shows it always seems to take us 45+ mins to get a single group through the FP line. Having that additional time with the rider swap for Group 2 to make it in line is almost a necessity in some cases. I'm not so sure I'd trust that a CM an hour+ later is going to remember us and allow us through past our FP window, or that another CM hasn't replaced them who won't let us on. I'm pretty sure someone on here actually posted that this happened to them where they were told Group 2 didn't need the rider swap since they had FP, then when Group 2 tried to enter the line after Group 1 returned, they were told they couldn't since they were outside their FP window. So yes, there can be a need for rider swap even if Group 2 has FP.