BOGUS!
I’ve never heard of a dead battery causing the airbag light to illuminate or flash. People disconnect batteries all the time and have no issues with the airbag light.
If the light is on or flashing there is a problem with the airbag system. The seller knows this and does not want to spend the money to fix it. Airbag repair can be very expensive.
Looking at the airbag means nothing. How do you look at an airbag? You should be aware that if the light is blinking the airbag is deactivated and will not function in a crash.
Walk away from this Miata. If the owner didn’t fix this who knows what else he didn’t fix. Find a car with no known problems.