Just because those companies say that doesn't mean the Trump administration doesn't think it can buy an exclusive cure for itself. That they can't doesn't matter as much as they appear to be trying.
The Welt am Sonntag report says "US-Präsident Donald Trump versucht offenbar, deutsche Wissenschaftler, die an einem potenziellen Corona-Impfstoff arbeiten, mit hohen finanziellen Zuwendungen nach Amerika zu locken beziehungsweise das Medikament exklusiv für sein Land zu sichern." which Google Translate assures me means "US President Donald Trump apparently tries to lure German scientists working on a potential corona vaccine to America with high financial donations or to secure the drug exclusively for his country."
And the German health ministry confirmed the Welt story to Reuters.
https://www.welt.de/wirtschaft/article206555143/Corona-USA-w...