Clients also self-report the amount of data transferred. That's not great in a community that fetishizes share ratios. I've heard an op say "there's no excuse for having a ratio less than 1", which makes as much mathematical sense as the parent who told my (math) teacher friend "this is [private school], no student should be below average".
You can theoretically verify upload/download numbers because the total amount uploaded in a swarm should equal the total amount downloaded, but there are all kinds of reasons why the numbers wouldn't match. Maybe a client lost connection and couldn't send its final announce. Maybe one client is sending bad data (I'm not sure how that is reported, might be implementation specific). And clients only send transfer total updates when they connect to the tracker to change status or request more peers, so every client will have a different degree of staleness.
Even if you can tell that someone in a swarm is lying, who is your culprit? As long as they're not being egregious, there's no way to tell.
Maybe the context of the quote was in regard to a private tracker?
The "ratio" in terms of private trackers isn't always the real ratio of GBs uploaded or downloaded.
There are for example some private trackers that grant additional credit for longer seed-time or they declare specific torrents "freeleech" so they don't "cost" ratio.
In the end they are just some of the measures private trackers take to strengthen their network, but they lead to a confusing definition of "ratio".