So when this slang-using generation grows up and joins the real world they will become the real world, and slang will become acceptable there and everyone will understand it. I fail to see the problem.
In many industries getting a job early and having work experience is very important, and your employers and colleagues will all be older. Being able to communicate effectively outside your peer group is critical for many jobs. Those that have the skill will benefit. Those that don't could easily get left behind.
Inability to communicate effectively outside your peer group doesn't matter much if you just program and don't try to talk to VCs, bankers, customers or potential mentors.
Two groups that would never use slang!
The author of the article seems to have a view of language that as subject to authority. This view fails to understand how language develops.
Slang is to use words that create "tribes" or sub-cultures, often to create bonds and connections with your peers, and often do not involve significantly new or specific ideas.
In each case, using your own dialect (slang or jargon) with your peers is one thing, but the ability to drop to a possibly less precise, possibly less efficient, but generally more widely understood common language is important.
That's what the article is about - the apparent inability of the target group (in this case teenagers) to use language appropriate to their listener.
Most half-way competent children of my generation used completely different language at school as compared with home. That's to be expected. What concerns me is the mono-linguality that seems to be emerging.
Yes, it is what it is, and probably nothing can be done about it, and maybe it's not a problem. But recognising that it's happening is at least linguistically interesting.
I find it amusing, but unsurprising, that something that gets people all upset (txt messaging and email "slang") is actually shown to enhance kids skills. I really wouldn't be surprised to hear the same about spoken slang.