The important thing is the context of the statement, which wasn't really clear originally. True, if you have the same singer and replace the guitarist, bassist, or drummer, and the new player is merely covering parts originally laid down by someone else, then it will sound essentially the same. But that's not what I got out of the original statement. The original statement seemed to be saying that while voices are distinct, instruments are not.
To me, a band's singer is, in more ways than one, the true voice of the band. You can change any of the instrumentalists and their instruments will sound essentially the same.
In the context of replacing someone and then seeing how much the result sounds like the band did on previous recordings, this is true. Instrumentalists can usually mimic the sound produced by another instrumentalist more easily than a singer can change his voice to sound like another singer. But we were talking about the changes which Spock's Beard has gone through over the years, specifically how they've had three lead singers, and to me the band still sounds like Spock's Beard.
Maybe because I focus much more on the instruments than on the singer. To me, a good singer is like another instrument, contributing part of the sound. One could argue that because voices are distinct, that the singer's voice is a larger factor, but I think that that's balanced by the fact that prog bands tend to have extended sections where there are no vocals at all. For me, the only time a singer really stands out is when they're very, very good, or really bad. Otherwise, they're just part of the mix, no more than the guitar, keyboards, bass, or drums. Many of my favorite bands have changed singers over the the years, yet still sound essentially the same to me.