When I prove, say, Cauchy's theorem about groups, it doesn't matter which concrete group I'm operating on to prove it. If I write a sigma to sum some vectors, it doesn't matter if the vectors are coordinates in 3D space, functions defined on an interval, or just numbers. Mathematical abstractions are pretty much the only ones that don't leak.
What, that you have to understand summation to understand what the sigma refers to? Well, duh. That doesn't make the sigma a leaky abstraction, because the sigma is not an abstraction at all! Not even a trivial one. The sigma is notation. And it's hardly news that you have to understand the concepts behind a notation to understand notation itself.
The author seems to misunderstand what an abstraction actually is.