That statement seems wrong in the strict sense, since the "Havlak" benchmark from your link has a more extreme difference between Crystal and Nim and was not included in the article's tests.

The word "bias" has several meanings. One of them is an accusation that those two benchmarks were chosen specifically to produce the desired outcome. Considering the above, and that Base64 and especially JSON parsing are very common, I think it would be fair to allow for the possibility that the article's author chose them accidentally and not with any intent to deceive.

FWIW I was surprised by the article's focus on interfacing with C, something I have not done in 20 years of programming, and was wondering if it was a (possibly subconcious) choice to set up Nim to win.

Indeed. A micro-benchmark on JSON parsing is a very misleading way to compare languages.