Why would someone care about their testosterone levels? If it's causing a problem shouldn't you are a doctor. If it's not then there is no problem in the first place.
Because an entire generation of online grigters convinced people that "big testo good, small testo bad". The same people who think baldness is a sign of low testoteron or that thin men have low t.
Can't say for much, but I do know testosterone is a drug (aka steroids) people willingly take to pursue a super muscular body type. Wouldn't be surprised if people tried alternatives to get the same effect, especially since onions are technically "natty" (natural)
There's probably also some folklore on test outside of bodybuilding, wouldn't be surprised if toxic influencers promoted it