It is theoretically correct but in the real world and in practice it is not. There are two reasons why it doesn't work the way theory says.

First is the fact that if it were that easy to earn bitcoin through claiming "free" coins through airdrops, everyone would do it. And when large number of people do the same thing (claim free coins then dump them) the price of that coin cannot be high meaning the amount of bitcoin you'd earn would be very small. Therefore you can't really call it "earning bitcoin".

Second is the fact that what we call airdrop or free giveaways is no longer airdrop or free. You actually have to work for it. Free giveaways used to be simple faucets like this one where all you had to do was enter your address and claim 5 full bitcoins! Nowadays you have to actually "work" to get "paid" that new coin. That work could be advertising their coin on social media, creating videos, and so on but it is work and you are getting paid. The payment is low as well, just as I explained in fist part.
So if you want to spend time and work to get paid, why not spend that time more efficiently and do something else to get paid a higher amount of money and use that money to buy bitcoin?

P.S. Keep in mind that exceptions may apply to what I explained. You could find some token that would give you a better reward but exceptions don't make the rule.