The "It's just JavaScript, so the frontend guys can work on the backend" line has never made any sense to me.

I think it's aimed at HR/recruiting.

Companies tend to think in terms of specific skills when hiring. Something like: We use PostgreSQL, so we will write "3 years PostgreSQL experience" in the job description for a senior backend engineer position.

Unless engineering is explicitly involved in the hiring process, HR will drop your resume automatically if you say "5 years of MySQL and SQLite experience" with no mention of Postgres.