They are wrong because concentrated sulfuric acid is only about 98 % pure.
Concentrated sulfuric acid is really a 2 % (m/m) solution of water in sulfuric acid.
Although it is possible to make 99 % sulfuric acid, H₂SO₄ tends to decompose at its boiling point:
H₂SO₄ → H₂O + SO₃
The best you can get is 98.3% acid. This grade is more stable, so it is what you get when you buy "concentrated sulfuric acid".
The misconception about purity probably arises because most samples are almost colourless. They "look" pure, like water does.