Homemade Vinegar Safe?
Yeast eats sugar and makes alcohol,
Acetobacters eat alcohol and make acetic acid which is vinegar.

Both alcohol and acetic acid inhibit other microbes. Once your solution is alcoholic or acidic enough there is no risk of mold.

However- this recipe does not include any yeast or acetobacters, it relies on getting the necessary microbes from the air and the surface of the pineapple. During that initial period there will be a risk of some other microbe taking over and spoiling the batch.

Personally I would take out a little insurance and include a pinch of yeast in the initial mix.

If you have an old bottle of undistilled vinegar around you might have a colony of acetobacters (called a "mother") growing in it already. This wouldn't be necessary to prevent spoilage but would speed up your vinegar production greatly. You would add a piece of the mother after most of the sugar had been converted to alcohol. After your first batch you could use the mother in future batches.

This sounds like an awesome experiment and I'm going to have to try making some as well.