3 Questions about the SES light not going out after a repair

01 Silverado 202K miles.. No SES codes last 4 yrs. Noticed trans not going
into first..taking off in 2nd. 2 days later while merging into x-way traffic;SES
light illuminates.Code generated: P0751.(shift solenoid A). Replace shift
solenoids A & B (sold in sets). Replace trans filter,fluid and gasket. Then I
realized I never disconnected the negative battery cable. I then did the
former for 15 minutes. Upon restarting the SES light stayed on. I then test
drove and it is performing like a newer trans..auto shifting throughout all 4
gears and shifting into all gears on demand. the only code showing is in the
history and its from before the fix. My questions for discussion are; 1)
Should I have test driven before I attempted to reset the computer by
removing the Neg. cable? 2) Will this reset on its own after some number
or cycles? 3) Why did disconnecting the battery not reset the memory or is
this a misconception; that it clears using the disconnect method?