It's a bullshit argument.  If it changes the security model at all, even slightly, it's a change in fungibility, and thus a hard fork because fungibility was altered.  They only attempt to use this poor argument because bitcoin isn't even fungible (and thus impossible to be money) in the first place.  So the Luke Jr "soft fork" hack is really a hard fork.  You'll notice Jbreher stole this argument of mine and used it to try and promote Bcash lol.

It's getting old watching these people in cryptocurrency trying to raise and lower levers up and down to do arbitrary changes at will exactly like Keynesian central bankers while pretending these things are "decentralized".
They're completely centralized on both the development front and mining front.  This stuff is all a complete joke decentralization-wise compared to physical metals.