Well the RP2040 has a QSPI controller but it has only a single channel that is normally the "boot device". If you bootstrap via SWD (or USB) this device could be at least a QSPI RAM, but writes would have to trapped and implemented in the HardFault handler which is of course very slow compared to internal SRAM. The RP2350 adds a second QSPI channel with QSPI bus (just an additional chip select pin).
TBH I don't know the details of how it works. I am not at all an electronics person. But did you read the article? All I'm saying is that it is possible to add more RAM to a Pi Pico. That, to me, seems to falsify the statement that it can't.
