As someone who has been developing for WP for over ten years, no, it's not difficult at all. It's just a very minor about of planning, and working with a process.

I believe the point in the article was about changes to the website itself. So what's the process here? Let's say you want to move some boxes around on the site, while another person writes a new post. You've got the change in dev and a new post queued in production - how do you merge that? Custom, partial db sync?

Theme changes don't affect the content DB. You'd make your change in dev, verify it, then change the prod theme without worrying about content.

Wordpress is PHP (and html/css/js/etc)and SQL. WP does not inherently do staging, so you set up a dev environment. You sync content from prod to dev, make code tests on dev, then copy them to prod. It's not hard.

No, I get it, but that's not what I meant. I meant changes to page layout/design which is in the db.

Unless you have people making multiple layout changes at the same time you shouldn't have collisions. It's 2019, you don't need to lock an entire DB to make changes to tables. You can sync those changes while content changes are being made if you so choose.