Well there is a valid point to that view. Usually web developers start out with front end development. And, if you’re only starting to learn then I think it would be ridiculous to try to learn the backend at the same time. Usually, developers shift their focus to the backend after getting comfortable in the front end, and eventually become a fullstack developer.
That’s certainly how I did my developer journey. But this path is not suitable for everyone or in all situations. It depends on your job prospects, your preference, and the job market. Fullstack developers do a lot more work than frontend developers and if you go fullastack at the beginning of your career (like I did) then you are looking at joining startups and small businesses. Sure, they’ll pay you a little better than pure frontend or pure backend, but not by much. You will be overworked though, and overwhelmed as well because you’re still getting the work experience.
The other case where starting out as a fullstack is more feasible and less stressful is if you’re going to freelance. That where your collective knowledge of the frontend and the backend will come in handy and let you price yourself competitively.
That said, I think it’s good to focus on one area (frontend or backend) and get so good at it. It will allow you to work in bigger businesses and get enterprise level jobs that require specialized expertise. It’s always possible to shift to a different area later on.
In any case, I say get comfortable in the frontend and just have a basic understanding of how the backend works and why and when they are necessary. That should be enough to get you your first job.