I think you have to be careful about taking this advice out of context. We had just been through a very bad time (with 1.8) due to severely under-optimised SQL queries. I sense, on the whole, that things have got a lot better and that the database is *less* likely to be the bottle-neck.
However, you don't get something for nothing and we now have caching with its own bunch of potential issues.
Essentially, if you are running a large, critical system you (still) have to know what you are doing - or find someone that does. This isn't a criticism of Moodle it's simply the reality of running a big web application.