Not true... people have the tendency to subconsciously believe that everyone online is running their time or at least are only 1 time zone away, even when they know this is not the case.
And yet, I have been in multiple realms that successfully talked about "Wednesday sunrise" and "Friday sunset" turns as timings for movement, with people spread around the globe, for long periods of time, with very high response rates.
In addition, I, personally, have no trouble keeping track of when people online are in relation to me. I'm very used to dealing with the 6-hour difference between my time (EST) and CET, the 3-hour difference between EST and PST, and the 11- or 12-hour difference between my time and Hong Kong time (from when my wife took regular business trips there).
So unless you can produce some kind of actual research on the subject, I'm gonna have to call BS on your bold claim :-)