In all likelihood it did exist since vanilla, it’s just that nobody knew about it back then and it’s not documented or tested anywhere. The first mention of it was near the end of WotLK.
The video provided (Decimator) is poor proof; you would need to go over countless old vanilla videos and manually count the hits/misses while HS was queued or not queued, corrected for the amount of hit gear the person was wearing at the time, to arrive at any statistically significant data set.
So as far as classic is concerned this is a bug until Blizzard compares it to the reference and finds that it did indeed exist back then and is just one of the many quirks of the old game, like the silly Arcane Missiles bug they decided to keep.