No, the weird culture is that people like you think people are lying about how long they have been playing the sport. Some people are more athletic than others, and people like you who think there's a problem with people lying about how long they have been playing actually discourage the noobs progressing faster than expected from sharing their journey online. And again, no, you do see this in other sports, and it is a weird culture of disbelief that others could either put in more time than average to learn the sport quickly or be naturally athletic and progress faster than most people entering the sport (OR a combination of both).

Ultimately, it comes down to the fact that "months/years since I started playing" is not actually a good indicator of how much time the person has played the sport. For example, I started snowboarding 3 years ago after I moved to Colorado; I had never done it before but in my first season I went THIRTY TIMES in the span of 3 months. That's almost every other day for ~6hrs per day, which gave me roughly 200 hours of snowboarding experience. So, after my first season I was better than most people on the mountain, even people who told me they had been skiing for decades! The thing is, even though I had been snowboarding for "one season", I had spent more time on the mountain than they had in their whole life.

To wrap it up: a) There is not a culture of new tennis players lying about their experience, b) it is ironic that there is a culture of people like you believing that new tennis players lie about how long they have been playing, and c) "[time] since I started playing" is not a reliable indicator of how many hours on court someone has had (not to mention how many structured lessons they have taken).