It is true. This only applies to people subject to US taxation (citizens and residents) and it doesn't matter where they dump the coins.
Yes, you owe the tax whether you sell or not. It is the same for U.S. stocks when a company goes public, original employees of the company are often given stock in the public company and they owe tax on the value of the stock on the day the company goes public whether they sell or not. This normally means they have to sell some of the stock to pay the taxes due.
What if a fork isn't on an exchange on the day it's created? Does that mean it is treated as zero value? Most Bitcoin forks struggle to get on an exchange within weeks of their creation. Some never even get on an exchange.
This is from their FAQ:
"When you receive cryptocurrency from an airdrop following a hard fork, you will have ordinary income equal to
the fair market value of the new cryptocurrency when it is received, which is when the transaction is recorded on the distributed ledger, provided you have dominion and control over the cryptocurrency so that you can transfer, sell, exchange, or otherwise dispose of the cryptocurrency."
https://www.irs.gov/individuals/international-taxpayers/frequently-asked-questions-on-virtual-currency-transactionsI guess you would have to have a good justification for showing what the fair market value was even if it wasn't on an exchange such as any trading activity you could find on this board or elsewhere.