There's a difference between cryptographic security and quality. Cryptographically secure generators typically have (empirically verified) high quality from a statistical perspective, but they're not a general solution for all purposes. They're relatively slow, you can't seed them, and they don't typically have rigorous mathematical proofs of certain characteristics like equidistribution and cycle length.

The article doesn't suggest Math.random() should be cryptographically secure (although that is an option). Rather, it's a critique of the algorithm that's being used. There are better algorithms with no drawbacks that produce sequences you can safely assume have a statistically random distribution (even if the sequence is predictable and is therefore not cryptographically secure).