Their reasoning sounds plausible only if read from a "business weasel" perspective. If there's a market for RPis with more lower quality RAM, nobody is "undercutting" RPF; they're selling the products they want to, at the price they want to, and choosing not to offer big/slow/cheap OEM RAM configurations. And even if someone could undercut them on the exact same components - So what??? RPF may be a non-profit, but I am not!
The problem isn't resellers undercutting RPi. It's someone buying a 1GB RPi, doing the mod with 8GB of poor quality RAM, then selling it as an 8GB RPi at a modest discount.
The problem is that you buy the board and because the RAM is bad, you perform a return and find out it was a counterfeit RPi board that doesn't work for you.
Despite the AI issue, you can find RAM. Often poor quality recycled RAM with errors are stupidly cheap (and used in many products as-is). The board can boot with bad RAM, but run stable it might not, and you'd be more likely to blame RPi than on the reseller you bought the dodgy unit from
The fact that RPi took this long to do it would mean they're actually seeing a bunch of returns with the RAM swapped out.
It could also be that people are buying the 8GB units, swapping them with 1GB units and selling them as fake 8GB units, like those $20 "1TB" USB sticks you can find easily on Amazon.
Changing the RAM on an RPi isn't a trivial procedure - I believe it's PoP RAM so it's not something a hobbyist would easily do without a lot of specialized equipment and experience in being able to separate PoP RAM from the base CPU and reballing and such. So hobbyist wise, they aren't likely to be ones affected since few would have the expertise or equipment. But those trying to sell counterfeits do.