Bombshell in the context of that website's subject/audience. Not a bombshell to the UK or world generally.
I'd imagine continuing the current scheme would add a lot of regulatory and procedural complexity for Amazon, which they are unwilling to handle.
Isn't this something that was expected by sellers?
If they've believed UK Government rhetoric about how Brexit will be a smooth process that will enhance the UK's trading opportunities, this may come as a surprise.
There are (IMO) a reasonable number of UK citizens and businesses who haven't grasped the likely consequences of what's happening on 1/1/21
Instead they explicitly rejected that opportunity and carried onwards...
I think it could well be right that they're hoping for the (likely impossible) blink from the EU, but between that, the heavy political toll a US trade deal would require (it would have a big impact on the UK farming community+possibly other areas) and the expanding spat with China, it's really not shaping up to be a good period, economically speaking.
Because those who promoted brexit comprehensively lied to the electorate about the consequences of brexit.