The speaker first names Zayd and then corrects the report to Amr. The function is correction of a report: the arrival now expressly affirmed is Amr’s.
«جاءني زيد بل عمرو»
Zayd came to me—rather, Amr did.
The textbook’s note explains that “bal Amr” asserts Amr’s arrival while leaving Zayd’s arrival or non-arrival possible. The correction does not separately establish the categorical negative claim that Zayd definitely never came.
Retracting one assertion and conclusively negating it are different things.