A more realistic end of the story would be that the form refused negative numbers so he had to put 0 and got fired.
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But block mode terminals that did forms had been a thing for over a decade at that point. Not that this was likely at Apple. But there are definitely contemporary ways in which one could have been entering this stuff via a computer.
Indeed, an IBM 3270 could be told that a field was numeric. This wouldn't have the terminal prevent negative numbers. The host would have to have done that upon ENTER. But the idea of unsigned numbers in form data had been around in (say) COBOL PIC strings since the 1960s.
* https://ibm.com/docs/en/cics-ts/5.6.0?topic=terminals-3270-f...