At the risk of committing an opinion, I should suggest this. Apart from certain adjectives which are recognised as also having a standard adverbial use (‘fast’, for example), many adjectives are used in that way. The example you give is one among many. under the entry ‘scary’, dictionaries do not offer a separate entry under the grammatical category ‘adverb’. So it is not a recognised standard use of the word. In that sense, it is not standard British English, or, I think, in US English, though my impression is that it is more widespread ‘over there’ than ‘over here’.
But it is possible that at some future time it will become standard. That is only a prediction, based on the fact that, love the adverbial use of ‘scary’ or hate it, the intended meaning is glaring obvious.
One, at least, has made it into Merriam Webster: real
Collegiate Definition (Entry 2 of 5)
chiefly US, informal
: VERY, REALLY —used as an intensifier It was a good furnace all last winter, they didn't have a single problem with it: it ran real quiet … — Garrison Keillor
… the magazine isn't real sure who its readers are … — Tom Carson
I could use the adjective ‘big’ as an adverb.
During my stay in Las Vegas, I won real big at roulette.
That use of ‘big’ is common enough to have made it into Merriam Webster.
Collegiate Definition (Entry 2 of 3)
: to a large amount or extent won big
: on a large scale think big
: in a loud or declamatory manneralso : in a boasting manner talk big