My experience, as a person responsible for hiring engineers, is that if someone says this it's because they need a way of filtering out candidates, who claim some type of classroom experience in lieu of workplace experience. In practice, it's tough to know which classroom offered what level of practical exposure to skills like collaboration, ability to learn on one's own, ability to communicate a thought process around multiple alternatives, etc.
"Build a real world app first" is a somewhat lazy way of saying, "we can't take a risk on you, don't have time to train you, don't have a great interview procedure for entry level developers, but also don't want to miss out on a possibly good candidate, so can you do something that fits better within our evaluation models like create an app in a framework we can assess?"
It could also be that to some employers, that assessment model ("Can you code an app now?") is the only one that matters - a candidate who's good at computational skills will still not hit the ground running with the specific frameworks the company uses, and the company might not even have a need for those skills.
My bias is to say, walk away from such an employer. If they don't want to invest in your growth now, chances are they never will. If they don't have a strategy for assessing "raw smarts," chances are they will never get around to figuring it out.