Yes this is at least semi legit.
I worked at YVR for 6 years and I can tell you every morning there's a crunch at US customs. I'm not sure why they don't open it earlier, but they don't. It's also not open really late at night.
What happens is, all the cattle show up 3 hours prior, then stand in a line that doesn't move, so it just gets longer and longer. The real wait is getting through CATSA security. The actual border itself is a non issue for the most part, because it's well staffed. But you spend so much time on the other side of the wall that by the time you get through customs you gotta hustle to the gate.
If you have status at least you skip the checkin line, because I've seen that 6am line at UA and it's mental. Full flights to ORD/SFO and IAH thrown in, it's insane. Honestly, I would advise showing up as early as you can. If it's a major issue then you're taking a risk.
If you have nexus/global entry I'm sure it's much faster, but still can end up in a pretty big CATSA line.
I'm on a 6am to SFO on the 24th and with status an no nexus I'm looking at showing up around 330-430 depending on how fast i can get myself moving at that time of day lol.