1- How fast can that personal assistant get an arbitrary submission out the door?
2- How much hand-holding do you think Mr Corden needs?
I'm not saying KDP is the best or cheapest but the track record for media companies like TIME and WSJ looking to get topical books out ASAP why the mainstream is actually interested is KDP.
For more typical projects there are more options ranging from KDP to Ingram to Direct2Digital to even B&N but sometimes you have to go with the automated 900lb gorilla.
It doesn't get much faster than login, set terms, upload, hit enter.
Note the KDP does not demand exclusivity--you can go with KDP and still go wide--and like Ingram, they do POD for fast access to the dead tree pulp crowd.
Different strokes and all that.
Carry on.
Since you asked:
KDP is fine for what it is: a purely Amazon-based market for ebooks. If you think you might have readers that get books through B&N, Google Play, Kobo (very big in Canada and some other parts of the world, #2 to Amazon overall), Apple Books, etc. then KDP is not good, because it ONLY puts books on Amazon (an important market to be sure, but far from the only one). It also has all the ethical concerns of giving your business to Amazon like that, which may or may not matter to any given author. They also exert some fairly draconian price controls on authors, where they cut the rev-share rate to 35% if the author wants to experiment with pricing -- they do this for good reasons (protects the perceived value and a healthy market pricing range for books), but in a very heavy-handed way where Scribl's CrowdPricing achieves better results without the heavy force of taking away the author's revenue.
To your question on speed, first there's no NEED to use Scribl's assistants, they're just there to help if you want (and I'm sure you're right that Jez is unlikely to need their help). If you self-publish a book through Scribl today, it will probably be live on Amazon within 24 hours (add a day or so if you want Scribl to review and assign a free ISBN). Sometimes much less, sometimes a little more. It probably is slightly faster to go live going through KDP, but then if you ever plan to sell the book any place else, you have to go through the whole process again and keep track of collecting your revenue from all those different outlets (e.g., B&N's Nook press, Kobo's Writing Life, Google Play's book account (a real mess to setup, but then a pretty good source of revenue), etc.). I would never publish through KDP for that reason, even if not using Scribl. I'd go through a D2D or PublishDrive or some other distributor over KDP. But those guys, without Scribl's patented CrowdPricing system, are forced to take a much larger cut of the sales revenue, leaving a relatively small slice for the author.