Well...the guy certainly does not equivocate. On the other hand, he says the show was just shy of his 16th birthday. I think that, at the time, hearing Fountain (or, as he wrote, "a weird kind of medley with the heavy parts of The Necromancer thrown in") would have stood out as being particularly noteworthy. If you asked me about set lists from when I was that age, I might remember a couple things (e.g., I remember that, at the first Rush show I saw (on the GUP tour, they opened with The Spirit of Radio, and the speakers were such that the first half of the song was nearly inaudible on the lawn at the back of Irvine Meadows), but I wouldn't say I remember much of anything with certainty.
The 1/10/76 show was apparently the last date of the "Down the Tubes Tour," and it was at Massey Hall in Toronto. Despite how dismal they've always said that tour was, they apparently closed out with a headlining show, without more than 90 minutes of material.
The guy on the Rush forum criticized the setlist.fm set as it existed in February 2015, but it's now changed to look more like what he said. Looking at the setlist.fm edit history, the list was changed in May 2015, apparently based in part on the Rush forum posts.
Interestingly, cygnus-x1.net reports that Fountain was "reputedly performed on November 28th and 29th."
https://www.cygnus-x1.net/links/rush/setlists.php#CARESSOFSTEEL But it does not otherwise comment on the Massey Hall show being a headlining set that was longer than the typical set.
There are certainly indicia of legitimacy here. However, it's really hard for me to accept that, on the last night of an incredibly difficult tour, they pulled out a 20 minute "epic" that they hadn't been playing all along. It's not like they had abundant rehearsal time to get it ready for performance. If they'd been playing it at soundchecks leading up to that show (or if they did hardcore rehearsals for that show during the five days they had off before that show), you'd expect someone affiliated with the band to remember that. And, if they hadn't been sounchecking it or didn't rehearse it, that means that, 74 dates into the tour, they pulled out a song they hadn't played in 5-6 months (since the album was recorded) for its first live performance. Without a recording, it's hard for me to believe this.