I'm not aware of a non-PC compatible that had an 80486. In any event, real mode can address up to 1MB of memory, with 640KB available to programs. But I do think you are right. If all he had was DOS and QBASIC, he had no way to go into protected mode. He probably just dropped the 0 by mistake.

No, he means 64 kB segments, which is what he had to deal with in the menu based file manager he wrote in QBASIC. That much should be clear if you read between the lines (the goto based text adventures etc.).