Yes, but also with a bit of lag.
A dictionary would be all but useless, if it included words used for 2 months, and essentially never used in that context, way, or spelling again. That sort of means teenage slang probably shouldn't appear, and amusingly if it did appear in the dictionary, it'd probably be less 'cool' to use that slang.
And alt-spellings for 'being kool' aren't what I'd call a dictionary thing.
Put another way, slang is in the dictionary, but only long-standing slang. Such as 'cool'. I'd see it more as temp slang->more used->enters dictionary-> eventually the 'slang' note is dropped, it's just language.
I'd want to see a word used for a few years, before it enters. And really, some slang morphs endlessly, like a fire flickering and then going out. How do you capture that? Even online, it'd be out of date once published.
And lots of slang is colloquial. Do you care to add slang language used across dozens of regions in the US?
I recall 20 years ago, Oxford Dictionary took a literal physical trip across Canada, and made the Canadian Oxford Dictionary, filled with 2200 'Canadianisms'. Akin to what you're saying here, if people are using it, it should be in there. But these were things Canadians had been saying for decades...
Anyhow.
I guess my point is, the kid was wrong because transitory has no value in the dictionary. His special words won't be used by even him, in a year.