I think that statement is a little too strong.
A person with the right skills and equipment should be able to replace the memory but it won’t be quick or easy to do.
We have the schematic available so if there are enough address lines and the like there might be pin-compatible parts to upgrade to. You also probably wouldn’t need to change out other passive parts but if so that rework would be relatively easier, especially since we have access to the schematic.
The schematic says this uses Hynix DDR3 1866 1.35V BGA100 chips.
There are 4 chips with 256Mbx16 configuration. (512MB each)
So you would need to replace all 4 with 1 GB chips which will take a long while to double the RAM.
Even if you get the chips replaced you would need to deal with the software. Since these are 32 bit Tegras you might have issues with 4 GB RAM. So maybe you could replace just 2 with the larger size and hope 3 GB works. But either way you would need to put in some effort for the RAM driver I assume.
You will need an ARM debugger and possibly whatever special IDE they used on the Tegra directly such as IAR. Also If I was writing the software on this I would also want an Xray inspection to make sure it was soldered correctly. You don’t want to be banging your keyboard for hours/days to later discover that 1 pin on 1 chip isn’t connected causing the whole system to fail. But this verification costs extra money.
So if you don’t have an xray machine for circuit boards, full access to a wide range of ARM dev tools and software/experience, and some sort of oven/heat gun and skilled technician, and special software skills you aren’t going to be able to replace the RAM yourself.
However I bet there is someone out there who might try this eventually. It would make for an interesting post, even if they fail at it.