My friend joined a car forum, because she’s into cars.
...and...
Apparently the way she talked was too “proper” to be “girly” and people responded that her knowledge of car components was a “red flag” that she must secretly be a man.
With this last comment right here it sounds like she is fairly knowledgeable about cars and the men on that community see "car knowledge" as their personal domain and expertise. If (gasp!) a woman knew more than a man on a car thing, then the man would be emasculated!! Since she was demonstrating knowledge, and perhaps more knowledge than some men, the men could only accept they were "beating" by another man and therefor they themselves wouldn't be emasculated.
tldr; the men in that community are terrified of being perceived as "less manly".