Assuming it was actually an ancient strain and not some modern contaminant, the most likely explanation is that the yeast sporulated, which would protect them. The revived strain would not be exactly the same as the parent strain, but there would be some strong similarities.
I don't know the circumstances behind the discovery, but there are many things that could have contributed to the cells' ability to survive even without sporulating. pH of the liquid over time, if it ever dried out and when and how, temperature stability, O2 levels, really just a host of factors that play into it. And remember, viability of cultures depends a lot on the media used and is determined in research papers by growing them for a certain period of time after chronological aging and then measuring growth in some way. These methods may be sensitive enough to note that a single cell is growing (they may not), but when the data are presented they are shown as a fraction of full viability and the plots trail downward over time. So it goes form 100% to 0% on a single plot - anything below 5% is pretty close to 0% due to the size of the plot and the line thickness. Anything at 1% is virtually indistinguishable from 0%. Imagine what 0.0001% looks like - that's still a lot of viable cells
Finally, there is the phenomenon of "gasping". Some cultures will start to grow again, long after the nutrients are depleted and when the plots show they are "0% viable". This is not well studied, but I would speculate that the viable cells are scavenging from the dead cells, and this could create new cells in preparation for sporulation.