While this may be true seven years ago, today (2016), if you look major retailers like Amazon, Walmart and overstock.com, they all ask for login before proceeding with payment. Best buy has a both guest checkout and login page before payment. Something has changed over the years and is not yet captured here.
I've actually backed out of making purchases on walmart.com because I didn't want to have to create an account. I always use the guest checkout if available, and only create an account if I really want a product on a specific website, or if I've purchased from the same company a few times already.
Amazon has many other services (including Prime) that require an account, so it makes more sense for them to require accounts than it does for any other online retailer.
I do the same for smaller sites on my second or third time buying from them. But the first time, if it's something I really want, I generally use a temporary email account anyway -- again defeating the purpose.
The thing is most people aren't Amazon or Walmart (I don't know overstock.com). People do have a relationship with Amazon, so their login makes more sense, simply because of the amount of repeat customers.
Most sites aren't Amazon though, they only get the customer for this one particular purchase, because they happen to be the cheapest, or the one with the item in stock.
If anything has chanced it's that your checkout flow needs to be even simpler, so it fits better on phones.
A few years ago (but still a few years after Jared Spool's article) I noticed that zappos.com required registration for purchase. I was super-curious about this because at the time, Amazon had recently bought Zappos and at the time, Amazon was not requiring registration.
I asked Zappos about it, and eventually got to someone who could talk to me about why--the answer was that they get so many returns that the overall customer experience is better if they know all your details up front. (I'm not sure why that's true but that's what they said).
They didn't give details (and I'm sure they wouldn't), but when I think about how I use Amazon now, I return a lot of stuff.
I personally dislike forced login/register but I've seen it work for some businesses.
In particular, the strategy I've seen work was to put registration early (but not too early) in order to get a contact email address and to follow up with people that drop out of the sales flow. Often people just need a reminder.
I enabled "email on incomplete checkout" that in a shop I own, and the response was very mixed. Some people were seriously offended, that we sent out a very friendly worded reminder email, that they left something in their basket (and had gone part of the way through checkout).
If they haven't actually A/B tested this, then perhaps nothing has changed and they could also realize an increase in sales if they did the same thing. Because they are big they probably wouldn't get as big of a boost, but again, without actually testing, it's only guessing.
I was looking for a specific product today that could only be shipped quickly by a few sites (no where I had an account). They all asked me to create an account. I ended up not buying anything.
