That's what the article is saying. You can quantify it by adding up the revenue from all the seats sold on a flight (or more likely, a route) and taking away the compensation paid for bumping.
The question I have is, if they don't overbook, do their planes fly slightly less full than other carriers or do they have fewer cancelations / no-shows etc.?
Hm? Presumably the comparison is between how much money an airline makes with overbooking vs without. And for this you would need to know a) how many IDB vouchers it's giving out and b) how many tickets it wouldn't have sold if it hadn't overbooked.
Especially the second part would require some fairly good idea of an airline's revenue management. Hence my question on how one can say that a particular airline is 'making money' by overbooking. Unless the author of the article (or someone here?) has access to fairly confidential information about various airlines.