You're committing a fallacy of equivocation. "Y2K" has two distinct meanings:
1. A software bug related to date handling that could cause incorrect behavior that was unpredictable in the specifics but bounded in the kind and extent of damage it could cause.
2. A software bug that could cause the collapse of society.
You might or might not remember this, but prior to the turn of the millennium there were plenty of people regularly talking about Y2K using the latter meaning. When people say that Y2K was a hoax, they're saying that the second meaning was not something that was ever within the realm of possibility, not that Y2K would not have caused any problems whatsoever.