Nonsense. You can absolutely teach yourself to optimize low-level code if that's what you want to do. The amazing thing about today's world is that all of the incredible resources available for learning just about anything in science and engineering to anyone who has the time to consume them. And you can work on open source projects to build your native skills.
The difficult part for you will be obtaining an interview. Because all of the other candidates will have CS degrees, and increasing numbers of them will have graduate degrees. You are at a disadvantage.
The main difference is that more resources are now publicly available to everyone to look up on the internet which was a different story 40 years ago. This is why companies at the time hired grads that can re-implement closed-source software from scratch from a reference spec or by reverse-engineering from another company to stay ahead. This has happened with device drivers and closed-source compilers.
To some extent open-source has removed the cost for reinventing libraries, reverse engineering and there is little need to study specific elements in a subject to solve such a problem when you can grab lib_whatever or a free compiler for a language.
You are right that in interviews at famous companies (FAANG, asset management companies, aerospace / embedded systems companies) being self-taught here isn't enough. Instead, they require specific certifications and they look for the graduate with a strong engineering degree rather than someone who is self-taught.*
Right now in 2019, I would do both.
*Having a serious open-source project or significant contributions is actually a huge advantage over recent graduates.
The other difficult part is to put aside time and money. I mean the information is most likely free but you still need food. Food is not free. Then you have to work so you can buy food. If you work, you will have less time to study by yourself.
In this sense, UBI is a godsend. But of course, we assume all actors are rational and self-disciplined.
The difficult part for you will be obtaining an interview. Because all of the other candidates will have CS degrees, and increasing numbers of them will have graduate degrees. You are at a disadvantage.