I'm not sure if it's true. It depends. Some stuff, for sure, like 3d printing tiny models of ingame characters, is something you can't just go out and do at scale without experience.
But there's a lot of 'artbook' and 'wallpaper' and 'soundtrack' stuff. i.e. 100% of things that they already have. This project on its kickstarter already displayed lots of art and wallpaper material, and they launched the game obviously with a soundtrack.
In short, shipping that is going to a local printer and asking to make a booklet or a wallpaper, in color. They give you a price and you inflate it by 10x. Wallpaper and booklet costs $3 to print, add a soundtrack and $3 in shipping (average, some will be 50c, some $5), and turn it into a +$25 reward. Get 1000 backers like that and you're netting close to $20k for relatively little effort. Even some ridiculous overhead and you still net $10k for stuff you can pay a highschooler a few hundred bucks to create. That $20k can pay a 3man passionate noodle eating team that's working on this parttime for months, and that's with a thousand backers, this team had a few thousand, actually I took a look and it seems that rewards are about half their campaign. It's really scrappy but it happens and it sometimes really works out.
Definitely not something that works for every project, in fact I see it fail more often than not. But it's not a priori a bad idea, I just don't think everyone thinks it through. Some devs create games and then have a mini comic booklet as a reward that increases the price by $5, or hell just include it for free because hey, kickstarter. That requires writing, printing and shipping, the costs don't make sense at all.