It means that YVR (the airport) offers this service to airlines at YVR that choose to pay for it. YVR advertises it on their web site to induce pax to plan connections via YVR (and YVR gets a hefty airport use fee, so this is great marketing for them). Whether any 2 airlines interline together on separate tickets is a matter of policy. AC will not interline on a separate ticket, for example (and documents this). You need to check with Frontier as to whether they interline at YVR (90%+ chance that they do not). Lots of other airlines do (AS and AA interline to CX, for example).

Without interlining you do need to clear customs in Canada and meet the check-in deadline for AC (as if you were originating at YVR). The deadline varies by the type of ticket you have on AC, but 90 minutes is not going to be enough.