Something like "oh, you're using map() and filter() in your C++ program? you're not doing actually doing FP unless [...]".

See https://news.ycombinator.com/item?id=33438320 for an example of this exact attitude.

Is that gatekeeping? It reads to me like they're saying it's not enough to just use a few functions to it FP. Their point seems to be that it's more than just using some functions, which seems reasonable.