what is the active ingredient in cymbopogon citratus that repels mosquitoes?
I think the problem isn't a hoax; it's just that people are overly excited about 'natural' as opposed to synthetic remedies.

C. citratus might be hazardous - the active ingredient is citral α-pinene. 

C. nardus might be hazardous, too - the active ingredient is citronellal or citronella. [There's confusion, in the article, about whether citronella has other plant chemicals in addition to citronellal.]

They're both partially effective as insect repellents. 

"It is commonly assumed that plant-based repellents are safer than DEET because they are natural. However, some natural repellents are safer than others, and it cannot be assumed that natural equates to safe."

This info all comes from a 2011 scientific article about plant-based insect repellents:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3059459/

Here's a link to a table with the results, summarized below:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3059459/table/T1/?report=objectonly

And a table with possible toxicities:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3059459/table/T2/?report=objectonly

Citronella has found its way into many commercial preparations through its familiarity, rather than its efficacy. Citronella was originally extracted for use in perfumery,..." 

C.nardus is at least somewhat effective in 3 lab studies; citronellal    is the active ingredient.

C. citratus - the active ingredient is citral α-pinene.  Again, it's at least partially effective in 3-4 studies, e.g., "74% protection against An. darlingi for 2.5h"

Here's a scientific article from 2011 about the scientific basis for the therapeutic use of Cymbopogon citratus, stapf (Lemon grass):
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3217679/

But it doesn't mention 'insect repellent' among its possible uses!:

"Studies indicate that Cymbopogon citratus possesses various pharmacological activities such as anti-amoebic, antibacterial, antidiarrheal, antifilarial, antifungal and anti-inflammatory properties. Various other effects like antimalarial, antimutagenicity, antimycobacterial, antioxidants, hypoglycemic and neurobehaviorial have also been studied."