car shut off but battery is fine

i have a 2016 hyundai accent with just over 30,000
miles. i was cleaning it today with the radio on and
the doors open for about an hour and a half when
all of a sudden it the lights inside started flickering
on and off and then it died. i tried restarting it and it
just made a clicking sound. i let it sit for awhile and
had unhooked the battery. when i got everything
back hooked up (about 30 minutes later) it ran fine.
no clicking, no warning lights on, nothing. i took it
to autozone and the guy wasn't the most intelligent
and i had to show him where to hook up the cables
to check the battery. he said it's charged and said it
must be the alternator. but i'm wondering if i just left
it on too long and it shut off because of that?