I believe he means being able to hear the band members' fingers slide up and down the strings as they move to new positions. Listen to the middle/end of Windowpane. That's a good example. I think they're meant as gentle slides, not intentionally played notes. It is pretty loud at the mentioned part of Windowpane, however.
As for an answer, I just think Steven Wilson does a nice clean job on the albums. I don't remember ever hearing those "sliding" noises on any PT albums. As a matter of fact, I'm not sure which PT albums SW produced. I'm assuming all of them, but other than the Coma Divine re-issue, In Absentia, Warszawa, and possibly some other re-issues, I think that the production on Porcupine Tree's albums is kind of muffled sounding. You think of good headphone music as Pink Floyd, Led Zeppelin, with lots of experimentation using sounds in the left ear, right ear, but PT's albums weren't like that. The muffled feel made it feel classic, though. I think that's cool.