[refurb]

Concentration has a lot to do with it. More dilute solutions of HF may cause burns, but t it's unlikely to kill you.

Concentrated HF is another beast entirely.

[tossandturn]

And given that it was in an old dusty bottle packed away deep, it may have concentrated over time due to evaporation.

[refurb]

Acids tends to pull moisture from the air (hydroscopic) and dilute rather than lose water and concentrate.