Hmm, right. Technicaly this paragraph was correct. But I'm reading "crawling process" as "OS process where crawling happens". The remark was important in past, because when many spiders are executed in a single process they can affect each other, and debugging what spider causes a leak was complicated; later chapters explained how to find which spider caused a leak. 

But I don't see a point of keeping this paragraph now - of course, if a process uses a lot of memory it can affect other processes executed on a same machine, but it is trivial, and it have nothing to do with big projects.