The estimate of how many deaths might be undiagnosed might be speculation, but it has already happened in Seattle that COVID deaths were misattributed to seasonal flu. In both cases, the death and symptoms will look identical the difference being in the underlying virus causing the issues.

"In fact, officials would later discover through testing, the virus had already contributed to the deaths of two people, and it would go on to kill 20 more in the Seattle region over the following days."

The US has failed to do any significant level of testing given its population, so it's understandable why many people are not confident in the official figures, especially when it becomes apparent infections have been running unchecked for weeks before testing started.

https://www.nytimes.com/2020/03/10/us/coronavirus-testing-de...