You can imagine digital dollar as a stable coin in crypto. There are stable coins like us dollar in crypto world too. But that doesn't threaten BTC at all. The dollar will be controlled by Government, which defies the sole purpose of BTC, which is freedom and anonymity of personal space. No strings attached.
Why do you think BTC has come to this position over the year?
It's simply because Bitcoin gives us something which dollar can't provide. And BTC becoming more and more popular day by day. So I don't think this will create any major threat towards BTC.
Digital dollar or simple dollar, none of these can be a threat to the bitcoin. Even no stable coin can ever become a threat to bitcoin. Stable coins cannot replace bitcoin. If dollars or stablecoins were that important, then we never had bitcoin in the first place.
To understand if it will be a threat or not, first we have to understand how that digital dollar will work... if governments can print unlimited money with the digital dollar just as they love to do with real money, then it will be a threat to bitcoin. But if the money is limited just as in bitcoin then it will be something good for the cryptos ecosystem.
Any money which is controlled by the government can not bring a threat to bitcoin, no matter if it is in the form of digital currency or fiat currency. Also, there cannot be a fixed supply of digital dollars and hence it will not be inflation free.