Sure, but I can imagine ants that injected gasoline or vinegar (for example) would be unpleasant too - while having those substances on your skin is fine.
Formic acid is a relatively weak acid (if I can read wikipedia correctly), much more like acetic acid than sulphuric acid.
Yep, from what I can tell it's about 10x stronger than vinegar at the same concentration, but about 1/7 the strength of citric acid (lemon juice). It's often use in wart treatments so it's not too bad in small concentrations.
However, it's flammable and can cause acid burns in the concentrations they're talking about in the article.
Formic acid is a relatively weak acid (if I can read wikipedia correctly), much more like acetic acid than sulphuric acid.