Kernel is an overloaded term and actually there's two different usages of the word in the article itself: one referring to the kernel operator[1][2] (the primary subject of the article) and another referring to the kernel of a homomorphism[3] (essentially the complement of a subspace in a coordinate system in the linear case).

[1] https://en.m.wikipedia.org/wiki/Positive-definite_kernel

[2] https://www.cs.toronto.edu/~duvenaud/cookbook/

[3] https://en.m.wikipedia.org/wiki/Kernel_(algebra)