I can't speak for WP Engine devs specifically but for Wordpress customization developers, the ones I've worked with were just absolute amateurs. The people doing this are mostly untrained people hacking together stuff layer upon layer until it kind of worked. Don't even ask about using version control. I don't want to say this applies to every Webdev but it attracts a group of people that aren't too much into IT but want to quickly learn programming to have a job.