POSIX and S3 are two doors into the same store. A file written through DirectFlow is readable through S3, and the reverse. One namespace, one set of bytes, no sync jobs and no second silo to buy, power, and reconcile.
The lake holds the data and the training tier holds the speed, so a copy job runs between them forever. That copy is capacity you buy twice, a window where the two sides disagree, and a pipeline step that can fail at three in the morning.
Object performance here is a function of which tier you point S3 at and how many Directors you run. Both are decisions you make, not limits the architecture imposes.