You think if a sysadmin does not understand the topic a software engineer magically will? I think the biggest obstacle of understanding systems the level SREs do is lack of proper documentation and formal education. This is why Google introduced the SRE university and also created the following doc.

https://landing.google.com/sre/book.html

What I mean by proper documentation is that you do not have all the knowledge in one place. I have spent countless hours of preparing to SRE interviews by reading source code and hunting for additional write ups that explain various OS functions, file systems, networking details. The amount of text you have to go through to get the knowledge is intense. The best way to become an SRE is to be hired one at Google or Amazon and just learn from your senior co-workers. Catch 22. :)