They don’t mention the winning one: A single short checkout. Purchase funnels are filled with account creations, newsletter sign-ups and other crap. I don’t want an account, I want to give you money.
I would suggest that the idea is that losing X% of first-time customers isn't such a big deal if you can increase LTV by Y% through marketing efforts otherwise not available to you.
That being said I don't really think the average marketing team has thought it through to this point (or measures the outcomes). But nevertheless it's perhaps why they see positive results.
It's a good way to battle people otherwise preferring to use a platform they already have an account on (e.g. eBay; Amazon) where you lose more on commission.
If my SO or I are talking about something there, it’s an almost certainty that one of us will press thumb to purchase. It’s too easy.