It's the truth, no cryptocurrency or currency is perfectly stable, including stablecoins. But the change in price of stablecoin is very tiny and it's acceptable. So stablecoin is safe to hold during a market crash. Most of stablecoins prices are pegged to USD, and you know that USD price is changing every second ( the same as others, that's why we have forex trading), so it's no way to keep stablecoin perfectly stable.
if you pay attention to stable crypto changes occur when bitcoin moves up and down. when bitcoin falls then the stable coin will increase in price. because some people sell assets and move to USDT and other stable assets. that is an old trick that is often done by investors and traders in the crypto market. its movements certainly cannot be fixed. because the value of fiat money in the world also has an exchange rate that is constantly changing.