If I understand it correctly (and memory serves): ATAPI is just the protocol used for ATA, and since SATA is just a different kind of ATA (Serial ATA), it can use ATAPI as well. Although SATA connectors are different and it works in a different way for HDDs/SSDs, it doesn't matter for optical drives - it's ATAPI for all of them.
See also Wikipedia:
(Was a little surprised to read that these drives basically "speak SCSI" but it makes sense given the history.)
IMO the answer to your question "Which one is true?" is "Both of them are true".