My take is that there is a wide gap between informal discussions of UB, such as blog posts, and the way compiler writers think of them. So I'm leaning towards the "obvious to researchers" option.
Just to be clear, Ralf Jung might be writing for a popular audience with this blog post, but is certainly a preeminent research of this stuff.
I guess I hope some writes a "back to the basics" blog post of this stuff at some point!