The confusing thing is that the team behind SHAttered did choose a prefix - they just had to choose a common prefix rather than two different prefixes (choosing a common prefix simply changes the initial state used by SHA-1). So I'm hoping that my post clarifies this point a bit.

The SHAttered attack (classical collision) can be used in practice - for example, my project https://github.com/nneonneo/sha1collider exploits their collision to turn any two PDFs into documents with identical SHA-1 hashes.