I would say perhaps the key difference here is we're talking about cosmological scale observations. Yes, a few quasars might've had to pass through some gas clo

I would say perhaps the key difference here is we're talking about cosmological scale observations. Yes, a few quasars might've had to pass through some gas clouds or an asteroid belt or space dust etc. but at the scale we're talking about, that's probably not going to skew the numbers too much. There's 190 quasars being measured here. Presumably more observations can be made over time and with other quasars involved but I don't think these scientists need more data to be confident in what they're seeing. If you think about a pebble getting in the way of old faithful, it's not going to make much difference if you're measuring how much water sprays out over a hundred years. Now you have 190 old faithfuls and they're all spraying out water in pretty much the same way, you don't really need to account for that pebble too much.