It's more of a precautionary statement than anything. The majority of people during pre-Smash 4 speculation (myself included) were guilty of saying "X character is definitely in!" and "Y character is definitely not in!" and then ended up being proven wrong when these characters ended up getting in/not getting in. It causes us to be less flexible with characters and too reactionary when someone said something about a character's chances that may not reflect how we feel about it.
I remember stating over and over again how Mewtwo and a DK character were guarantees (I was especially confident of King K. Rool's inclusion) as well as Ice Climbers, Lucas, and Wolf. I, like many other people, got burned badly when these "guarantees" didn't happen and also got shocked when characters I had ruled as "impossible" got in (most notably Rosalina). While the roster still turned out very good all things considered, I will never make the mistake again of assuming X or Y character will definitely be in/not be in.
This is the problem with a disappointingly large portion of the speculation community; the belief in shoo-ins, guarantees, and impossibilities. In other words, absolutes. Too many times I've seen people claim that a character was certainly, without a doubt, 100% going to be in/
not going to be in. If there is one we can be certain about, it is the uncertainty of the future. We, as speculators, simply do not know what is going to happen in this game, which is why speculation exists in the first place. Despite this fundamental principle, there are far too many people who claim that certain things
will happen, as if it is an undeniable fact. This talk of absolutes creates the false sense that there are no other possibilities, even though there are infinite possibilities beyond our comprehension. This, in turn, creates dissent within the speculation community.
Over the course of speculation history, there have been far too many things treated as guarantees: the Gematsu leak, Chrom, Chorus Men, Mewtwo, Ice Climbers, etc. Too many people acted as if these were simple truths that were beyond question. In such a community, those wise enough to recognize the existence of unconsidered possibilities were often made the target of ridicule and insults, the most common of which being "denial." This insult in particular stands out as being completely out of place in a speculation community; you can't really be in a state of denial (at least, in the sense we use it) if what you're denying is not a confirmed fact. But, of course, so many labored under the delusion that these unconfirmed facts were reality, continued to deny the existence of other possibilities, and attacked those that did not share their delusion. This sort of behavior destroys the basis of speculation and, frankly, just ruins the fun for everyone.
In short, there are no guarantees in Smash. There never have been, nor will there ever be, and the community would be much better off if it realized that.