[Scali]

Yes, especially somewhat later in the life of the 8088, most applications would demand quite a bit of memory, and the 640k configuration became standard for the later 5160s and most clones. So I wouldn't be surprised if 5150 owners also upgraded the memory on their machines. The AST Six Pak Plus card was quite a popular upgrade. It could add up to 384k, so if you had 256k on board (fully loaded later model 5150), there's your 640k.

If you were unlucky enough to have an early model 5150, the board could only take 64k. That limit was however because 4116 memory chips of 16kbit capacity were used. The board could be modified to take the later 4164 chips of 64kbit capacity, in which case it would take 256k. Alternatively, you could use two memory expansion cards to bring the memory up from 64k onboard to 640k total. Although I doubt many people would use such a contraption in practice.

Early model 5150s also required a BIOS upgrade, as a bug would prevent the BIOS from detecting more than 544k of installed memory.

So in theory it is possible to have a 5150 with 640k, and no doubt such machines exist in the wild. But it's probably far more common to find a 5155 or 5160 with 640k. So just like with 8088 MPH I guess that's the 'practical' target of the demo.