Raspberry Pi is actively preventing enthusiasts from upgrading soldered memory to a larger configuration even when the replacement chip is technically compatible. The company says the restriction is intended to discourage unofficial high-capacity resales.
Their cost is higher than the official one, but they can still undercut the official one and make money. That’s why it works.
Personally I’m not aware of anyone actually selling these, so this may be a made up issue. Or it might be a case where they were sold on aliexpress or something.