There is good reason why the slot has yet to be activated.
To begin with, there are only around 4 drives that are likely to be fast enough. What this add-in drive cannot do is offer any performance level below the built-in solution.
The specification of the onboard solution is "what the PS runs at" and almost certainly some designers will want all of that available bandwidth. Ratchet & Clank from everything we've seen about it so far is going to be extremely demanding on that SSD.
Having an add-in drive that isn't up to spec, causing any kind of stutter or issue with the game is not acceptable and it'll be Sony who will have to sort this issue out.
The drives need to be tested empty, half-full and full to se if performance changes. The drives will need to be tested at low temperatures, normal temperatures and high temperatures - to see if that makes a difference to transfer rates.
They will need to simulate months an potentially years of operations to ensure the drives are reliable.
Although Sony are not the ones makes the drives, at the end of the day, as soon as Sony "put their name" to a drive, which they are doing be adding it to any officially compatible list, they have to be sure their name is not damaged.
