I don't want to enter this discussion so I'm just going to leave my view on this and my sources at the end. Not trying to convince anyone nor to discuss further on the topic.
First of all, the PS5's specs.
Sony talks about a custom SSD, and whats custom about that SSD is the Flash Memory Controller. The flash memory controller is what manages storaged data on flash memory and communicates with the host machine. What this means is that Sony developed "the brains" of the SSD, but not how it communicates with the host machine. Sony talks about at least 5.5 GB/s and a 12 channel interface. The 5.5 GB/s are the minimal data transfer speed the flash memory controller can achieve when sending data to the host machine. It sends all this data through a 12 channel interface which we can assume is not custom as we have more than capable consumer level technologies that can achieve more than what the PS5 does, so they're probably using an already existent one.
The PS5 uses Zen 2 cores, which probably means no PCIe 4.0, only 3.0. If we assume this SSD uses an NVMe interface, then the theoretical max speed it can achieve on PCIe 3.0 x16 is 15.75 GB/s. They're talking about "12 channels", translating this to 12 PCIe 3.0 lanes we can see that the advertised 9 GB/s are more than possible as the maximum speed using 12 lanes would be around 11.8 GB/s.
Now let's talk about NVMe.
The PS5 has NVMe expansion slots, and as such, we can assume that the main SSD will also use NVMe as it is an industry standard and it wouldn't make a lot of sense to mix and match different memory standards. NVMe explicitly works with PCIe, as NVMe is the standard which we can use to access non-volatile memory thourgh a PCIe bus. If the main SSD works under NVMe then it works uner PCIe standards, if it uses Zen 2 cores then the PCIe Bus is a PCIe 3.0 Bus, if this is the case ten the SSD's max data transfer speed is capped at 15.75 GB/s on 16 lanes, we know the PS5's SSD flash memory controller uses 12 "channels" which we may assume translates to 12 PCIe lanes, then the speed cap is around 11.8 GB/s, the advertised speed (between 5.5 and 9 GB/s) is well between the PCIe 3.0 limits.
Then I'll conclude that the PS5's SSD works under PCIe 3.0, using an NVMe drive that has a custom flash memory controller that uses 12 lanes and achieves speeds higher than 5.5 GB/s but not higher than 11.8 GB/s (unless it uses more memory lanes or they change their processors to Zen 3 architecture which works with PCIe 4.0 and could mean higher data transfer speeds).
( Flash memory controllers:
https://en.wikipedia.org/wiki/Flash_memory_controller ; PS5 specs:
https://www.techradar.com/news/ps5#section-ps5-specs ; PCIe:
https://en.wikipedia.org/wiki/PCI_Express ; NVMe:
https://en.wikipedia.org/wiki/NVM_Express ; Zen 2:
https://en.wikipedia.org/wiki/Zen_2 ; Zen 3:
https://en.wikipedia.org/wiki/Zen_3 )
