I have to agree with Daze - yeast are quite resiliant little fungi. A few months without food is nothing is nothing to a yeast colony. I've even had some wine yeast obstinately survive my attempts to kill them.
Personally, unless you gave the guy at your homebrew store a reason to believe that you had extensively tried to erradicate the yeast (i.e. cold crashing, sulphie+sorbate, Sterile filtration, etc.) I would have to question his advice on things in the future.