Snowflake actually has historically run as a multinenant service with access to say your S3 bucket, so the majority of data transfer is VM <=> S3 in the same Region (so included/free).
As a snowflake customer, you would submit queries (and data ingestion) as Egress pricing (roughly $.01/GB though assuming it was using the private link or similar) and then the smallish results come back as Ingress (which AWS charges for, we do not, but whatever).
I hope someone corrects me on the AWS side (I really thought that there was a way to do zero rating for same “AZ” third-party services), but it’s not fundamental.
That cost acts as a barrier if AWS launches its own Snowflake competitor. Basically AWS taxes you for not using other AWS services, once you are inside AWS.
It really does feel hopeless to run an open-source service that isn't running on an existing crowd. Perhaps that's one reason Snowflake is doing well.