I understand both sides here.
Hams use that phrase to let new people know that it's not necessary to run high wattage to shoot DX. And they are correct. As a former Extra class operator, I have worked worldwide on several bands with only QRP output (<5W), so it can be done. But as a CB operator, I can tell you it's easier to work the ham bands at low output than on CB, simply because you have all the CB'ers crammed on one small portion of the band. If a person gets into the "freeband" area of 11M, it does get easier to work DX with low output, because of the lower traffic. But from the stance of legality, on normal 40 channels, and 12W SSB output, it's not the easiest thing to do.
Can you work the world on 100W or less? Absolutely. Is it relevant to a CB'er making a fresh start? I don't think it's all that relevant, which is why I understand where 359 is coming from. Just for the fact that CB does not go to 100W to begin with. Sure, a peak and tuned CB can do 20-25W SSB, and that's all fine and dandy, but it's no 100W. And there is more traffic on CB too (due to the small bandwidth that CB occupies), that can cause the pileups that make working DX on the band more difficult.
I can see why this statement could be made when a person is considering running a large amp, to let that person know that it isn't necessary to run 1-15KW of output to work DX.
Want to work the world on less than 100W? Best way to do this is to get a ham ticket. This is the advantage of having a ham ticket. The ham bands is the ultimate "great wide open". Let that motivate you to get that ticket and enjoy it all.
All in all, I do understand why the "I can work the world on 100W or less" statement is made, but I do agree with 359 that in the CB world, it is easy why this can be taken out of context, and understandably so.
Just my .02 worth.
~Cheers~