> didn't realize it was not standard Slovak.
Just a tiny shift: it is standard Slovak, it's merely colloquial, informal -- like the difference between ahoj and dobry den, both are standard Slovak, but ahoj is colloquial, informal. _Hej_ is acually less informal than _ahoj_, people often use _hej_ in communication with someone they address as pan/pani, which would be impossible with _ahoj_.
Some fussy teachers chastise students for saying _hej_ in class, which is a cultural carry-over from the 19th century when the language of the classroom was much more "Biblical": if something didn't match, then it was "plain wrong."
The massive use of _hej_ all over Slovakia is one of the indicators that _ano_ was introduced from the outside, artificially, so to say, through the language of the Bible. The word was not "born" in natural Slovak.