I doubt it.
I travel to/from YYZ 2 - 6 times per year, not always on UA, but probably about 9 times out of 10. Never heard of anything like that. Have often checked in between 60 - 90 minutes prior to departure.
Check in cutoff is 60 minutes prior to flight for any flights to/from Canada, and I find that's very strict. According to this page,
https://www.united.com/ual/en/us/fly...t/process.html, which has the required check in times, as well as airports with exceptions to the general rules, nothing listed for YYZ. Meaning officially, the only requirements are the general - must be checked in and have bags checked at 60 minutes prior to departure, and must be at the boarding gate ready to board at 30 minutes prior.
On the
Important Notices page, they have the following generic information about US pre-clearance, but I'm pretty sure it hasn't really changed for years:
Traveling on flights precleared by U.S. Customs and Border Protection
If you’re traveling to the U.S. from one of the international airports listed below, we recommend you arrive at the airport three hours before your scheduled departure time. Flights from these airports are precleared by U.S. Customs and Border Protection, so please allow extra time to complete additional border security screening processes before you depart. When you arrive in the U.S., you’ll arrive at a domestic terminal.
We operate flights precleared by U.S. CBP before departure from the following international airports: Aruba (AUA), Bermuda (BDA), Dublin (DUB), Freeport (FPO), Nassau (NAS), Shannon (SNN), Edmonton (YEG), Halifax (YHZ), Ottawa (YOW), Montreal (YUL), Vancouver (YVR), Winnipeg (YWG), Calgary (YYC) and Toronto Pearson (YYZ).
Also given the amount of folks using Nexus/Global Entry, I can't imagine they would enforce a 90 minute cutoff through customs. I can often make it from curbside to through customs in 15 minutes or less with Nexus. And as noted, security and customs both scan your BP - security twice - at the ID check when you enter the line, and just before moving to the scanners/x-ray. I'm pretty sure that's just CATSA capturing wait times - I'd doubt if the airlines have access to it. Have no idea if the airlines can see when customs has scanned pax BPs. I suppose its possible.
I'd guess its a tactic to scare people into arriving earlier, and I wouldn't worry about it. On that note, do note that during busy times, without Nexus/GE, it can be a while to clear customs - don't know how long since they moved to the kiosks, but you used to have waits of 60+ minutes sometimes. I don't know if there is ever a time so busy as to require arriving 3 hours early - 2 for sure during peak days/hours without trusted traveller.