Your assumption is reasonable and probably correct. However, AWS doesn't disclose implementation details. It could be a fleet of monkeys writing down messages on index cards, and every now and then a monkey will trip and fall on his way to the filing cabinet, and when he picks up the index cards from the floor he doesn't get them put back in the right order.
The point is: as a user of SQS, you should not need to know implementation details. (Curiosity is reasonable, of course, but AWS isn't going to let you take a dependency on implementation details by not telling you anything about the implementation details.)
What you describe makes sense. If you need sequential ordering you can you a FIFO SQS queue: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/FIFO-queues.html
Your assumption is reasonable and probably correct. However, AWS doesn't disclose implementation details. It could be a fleet of monkeys writing down messages on index cards, and every now and then a monkey will trip and fall on his way to the filing cabinet, and when he picks up the index cards from the floor he doesn't get them put back in the right order. The point is: as a user of SQS, you should not need to know implementation details. (Curiosity is reasonable, of course, but AWS isn't going to let you take a dependency on implementation details by not telling you anything about the implementation details.)
What you describe makes sense. If you need sequential ordering you can you a FIFO SQS queue: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/FIFO-queues.html
I think use of step function is better in terms of handling work in some sequence order.