I don't disagree with you, and I believe you're probably teaching people many of the right things that will set them up to do well in the future. But...the tricky part about learning just concepts and not the skills is that they are quite abstract and its very hard for a junior to immediately see how they can applied in practise when getting their first job. They're also quite hard to demonstrate to an employer. Learning SQL gives you some tangible to do that can help you to learn something about many of those areas, maybe not too deep, but while also getting a hands-on skill that easier to demonstrate to an employer. The reason I think SQL is the right skill there is that its specific enough that its actually useful, but its generic enough that its widely applicable to many employers (rather than specialising on one specific tool, like say Spark).

For example, you might learn the theory behind internal combustion engines, but does that make you more attractive as a Mechanic's apprentice than a kid who's rebuilt his lawn mower's engine? Neither are 100% perfect, but the hands on experience might be easier to demonstrate that you can be effective on day 1, and you can learn some of that theory on the job. Idk if that's the best analogy, and I'm not saying it's 100% the case here.

I did a degree in Compture Science and learnt a lot of theory, its all super interesting and personally I still love understanding all of the theory and low level details behind software...but many of the DEs working in Enterprise don't have that background (or interest). And that's not a bad thing, they're just as competent data engineers as I am.

I guess it's a matter of what your end goal (and timeline) is. Some folks aren't looking to be a word-class DE expert, for some its just a job and DE is a booming space with above-average compensation. In that case, you're probably not inclined to sit in a class room and learn all the theory, you just want to land the job and get on with it. Nothing wrong with that at all, some of the best get-stuff-done engineers are like this. For others, even if they find the space fascinating and they want to be at the top, they might be more interested in one very specific hand-on area and they only need as much theory as they pick up day-to-day.

Anyway, I don't think your approach is wrong, and everything you're teaching is great for someone who is genuinely interested in the field of DE - but I also don't think its all absolutely necessary if you just want to get your first DE job.

RE. DE vs ETL developer; I think DE has become more of an umbrella term like "Software Engineer" which encompasses many different roles. An ETL developer probably falls under Data Engineering these days, its just a specific niche.