I didn't claim it was rocket science nor that developers shouldn't learn these skills; only that optimizing a database is out of bounds of what most would consider "basic relational database theory". But beyond that, lots of developers aren't interfacing with a relational database. In a past life, I worked on embedded electronics. Other people work on operating systems. Others specialize in compilers or graphics or etc. Databases are a good skill to have, but it doesn't mean everyone should train like a web app dev either.